One is bound with the bondage of craving, and the arahant has become released from this bondage, and become unbound.
From Iti 58:
This was said by the Blessed One, said by the Arahant, so I have
heard: "There are these three cravings. Which three? Craving for
sensuality, craving for becoming, craving for non-becoming. These are
the three cravings."
Bound with the bondage of craving,
their minds smitten
with becoming & non-,
they are bound with the bondage of Mara —
people with no safety from bondage,
beings going through the wandering-on,
headed for birth & death.
While those who've abandoned craving,
free from the craving for becoming & non-,
reaching the ending of fermentations,
though in the world,
have gone beyond.
The second noble truth says that the cause of suffering is craving.
But what's the relationship between craving and ignorance?
Have you heard of the old South Indian Monkey Trap (from this article)?
In Zen And The Art Of Motorcycle Maintenance, Robert Pirsig’s
bonkers-but-brilliant philosophical novel that turns 40 this year, he
describes “the old South Indian Monkey Trap”. ... The trap
“consists of a hollowed-out coconut, chained to a stake. The coconut
has some rice inside which can be grabbed through a small hole”. The
monkey’s hand fits through the hole, but his clenched fist can’t fit
back out. “The monkey is suddenly trapped.” But not by anything
physical. He’s trapped by an idea, unable to see that a principle that
served him well – “when you see rice, hold on tight!” – has become
The monkey needs to let go of the rice in order to free himself from his suffering. The way to end his suffering, is to end his craving for rice.
But in order to end his craving for rice, he must first understand how his hand is stuck inside the coconut. When the monkey overcomes his ignorance about how the trap works, he would let go of his craving for rice, and release his clenched fist. With this, he would be free from his suffering.
Also you can say that the monkey's suffering originated with his ignorance of the trap.
Knowing how the trap works, i.e. gaining insight and wisdom into the nature of reality, is the way to overcome ignorance.
(Illustration above: Paul Thurlby for the Guardian)
The Upanishadic "That thou art" is unrelated to Buddhism, because it tells you that you are actually that eternal Self. In Buddhism, all phenomena is not self and the self, being a mental idea, is not eternal or permanent.