it is the knowing and experience of no self / anatta. You've been told this and nothing compares to experiencing it as a certainty and truth. That the you you thought you were is not real and never was. Suddenly there is an existential crisis. If I am not real then what is the point? Serious anxiety and horror from this but it passes as a new reality asserts itself. The ego still exists but you're no longer trapped all in there and can see it separate from that which is aware.