Usually a line over a symbol for a set means the complement of the set. In this case the complement of Q is the set of all irrationals (assuming the whole space referred to is the reals).

It could also mean the closure of the set, in which case (considering it as a subset of the reals, and in the usual topology on \(\displaystyle \mathbb{R}\)) we would have \(\displaystyle \bar{\mathbb{Q}}=\mathbb{R}.\)