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In the Heart Sutra, Avalokiteshvara says to Sariputra
this Body itself is Emptiness and Emptiness itself is this Body. This Body is not other than Emptiness and Emptiness is not other than this Body. (translation by Thich Nhat Hanh)
What does it mean to say that Body itself is Emptiness and Emptiness itself is this Body?
In my understanding, this basically speaks to the following Buddha's expression (SN 22.90):
By & large, Kaccayana, this world is supported by a polarity, that of existence & non-existence. But when one sees the origination ... as it actually is with right discernment, "non-existence" ... does not occur to one. When one sees the cessation ... as it actually is with right discernment, "existence" ... does not occur to one. ... "Everything exists": That is one extreme. "Everything doesn't exist": That is a second extreme. Avoiding these two extremes, the Tathagata teaches the Dhamma via the middle.
The way an untrained run-off-the-mill person sees things and talks about things, is that anything, any object or phenomena - be that physical or social or economical etc. - either exists or it does not exist. Everything is very cut and dry. But in a wise one's perspective, things are not so simple, not so flat. The wise one sees infinite nuances. This is why the wise one avoids taking extreme simplistic positions about things being or not being a certain way.
Any form - meaning any object of our attention and discussion - is necessarily an abstraction, a subset, a vast simplification of reality. To assume that it actually "exists" would be to commit (succumb to?) the reification fallacy. Instead, a form is a phantom, a fiction, a conceptual overlay. Therefore a form is empty of existence in-and-off-itself. Form is empty. That includes any and all dharmas (phenomena, objects of mind) that some of the late Abhidharmists mistakenly reified. Like the five skandhas (the form/body, feelings etc). Or the Five Great Elements. Or the dharmas. Or even Nirvana itself. From Madhyamaka perspective, these are all abstractions, simplifications, generalizations.
At the same time, to say that things don't really exist - would be to fall into another extreme. Even though form is empty, we appreciate it for its explanatory value. The way we delineate the world into forms, into discrete phenomena, is not entirely arbitrary. It came from our real experience dealing with real things. So the forms we experience, however subjective, conventional, faulty, and incomplete, are rooted in the real nature. The forms do exist, at least in our subjective experience. They only exist as conceptual overlays, but these overlays themselves are not non-existent as overlays. Just like rainbow - there is no actual arch-shaped object in the sky, but you can't say that rainbow does not exist. It exists as a phenomenon. And that phenomenon - however illusory - has its basis in reality, in how things work. This is the "Emptiness is Form" part.
Some of the practical implications of "form is emptiness", is that we no longer get hung up on any one truth, any one method, any philosophical position. We see that they are all ultimately false. On the other hand, because "emptiness is form", we do realize that our choices have effects, and these effects often fruit as experience, and this experience will be rather very real for someone experiencing it! This realization of form-is-emptiness as the ultimate truth is known as the root of prajna or (special) wisdom. And this realization of emptiness-is-form as undeniable reality of experience or the conventional truth, is known as the root of maha-karuna or great (enlightened) compassion.
So at the end of the day we get very practical. Given that form is emptiness and emptiness is form, we do the best thing possible using the best tool for the job. If we need to cut through we cut through, if we need to skip over, we skip over. We don't get stuck beating our head on the wall.
Short answer - It is a description of the enlightened state of mind. It is an experience, not a concept. Asking for it's meaning is akin to asking for the meaning of an orgasm.
Long answer - I think where a lot of people run into problems is that they try to read the Mahayana canon like the Theravada canon. This doesn't often work as they can be very different in their approach. The Pali sutras are usually pretty prescriptive. They are most often read as a "how-to" guide for practice. Even when they are being descriptive or go into some sort of philosophical idea, they are generally pretty literal in their treatment. While you also find examples of this in the Mahayana canon, there is also quite a bit of poetic language the purpose of which is to describe in abstract or metaphorical terms some instance of enlightened phenomenology (the Diamond Sutra being probably one of the best examples of this). These are road maps and personal accounts from the journey to enlightenment. Trying to tease out some sort of philosophical meaning from these sutras would like using someone's description of a barking dog as the basis for canine behavior. There's no point in it. Any exegesis or anything deduced would be wholly meaningless and off point.
While the Heart Sutra uses Buddhist philosophical terms like form and emptiness, this sutra is not something we are supposed to understand intellectually. It's actually a sign post - something by which we can measure our progress. It lets us know we are moving in the right direction. The Heart Sutra is a way station, in linguistic form, where people on this journey can share a beer and trade stories. Only after reaching that destination do we understand the point it's trying the convey.
Even longer answer - We know what form is. Knock, knock on wood, right? We all experience that pretty easily. Emptiness is also something to be experienced. It's not just a philosophical idea. That's a little more subtle, but I hope that makes sense. What the Heart Sutra is getting at is the experience where the distinction between form and emptiness dissolves.
What if there are infinite explanations and meanings. Can also mean the 4 noble truth as follows:-
"emptiness is body" means "whatever results from meditation, habits, learning or karma" is "body" (some prefer to use mind", others prefer to use "form" which is "sufferings").
Body is emptiness means "whatever concrete things we see, touches, hear etc." is "habits, karma, meditation etc." Suggesting there is cause to these sufferings.
Further "emptiness is none other than body" and "Body is none other than emptiness".
These last 2 verses thus means that liberation is possible and training towards liberation from suffering is possible.
Imagine "Emptiness is emptiness, body is body"....means there will no liberation .. if not for the last 2 verses
