Ignorability (better called exogeneity) simply means we can ignore how one ended up in one vs. the other group ('treated' Tx = 1, or 'control' Tx = 0) when it comes to the potential outcome (say Y). The potential Y outcome of person i had they been treated or not does not depend on whether they have really been (observable) treated or not. We can ignore in other words how people ended up in one vs. the other condition, and treat their potential outcomes as exchangeable.