A depeg is the moment a stablecoin trades away from the value it claims. A cent either way on heavy volume is normal plumbing. A persistent or large gap is the market pricing real doubt about the backing.
Depegs are reflexive: as the price slips, holders rush to exit, which slips it further. Depth is what stops the spiral, which is why a depeg on a thin coin is far more dangerous than the same wobble on a deep one.