The Heart Sutra is of utmost importance in Mahayana because it declares the key realization that makes Mahayana distinct from a superficial "Sutra-Yana":
That in the ultimate sense all concepts are arbitrary, including even the concepts comprising Buddha-Dharma, that ontological reality behind concepts is by itself free from any discriminations, ...
The Sutra is rather famous instead of important because it was mentioned in one of the most popular Chinese Novel and Movie making, The Journey to the West.
In the centuries following the historical Xuanzang, an extended
tradition of literature fictionalizing the life of Xuanzang and
glorifying his special relationship with the Heart Sūtra arose, of
The "root" is a passage from the 18th chapter of the MMK:
諸佛或說我 或說於無我 諸法實相中 無我無非我 諸法實相者 心行言語斷 無生亦無滅 寂滅如涅槃 一切實非實 亦實亦非實 非實非非實
All Buddhas either speak of self or speak of no self. All dharmas’
true aspect, within this, there is neither self nor no self. All
dharmas’ true aspect is defined as mental activity’s and spoken
language’s ending.There ...