Definition of lockout
A lockout is when a card helps a player by making it impossible for him to lose. In other words, when a card causes one player to have 100% equity and the other player is drawing dead, the situation is said to be a lockout (you could say that it is now in the bag). When we say in poker that we are drawing dead, it means that our equity is at 0. When you are drawing dead, it means that you are all-in with a draw, but even if you hit your draw, you still won't have the best hand.
You are in the small blind with AK in the heart. The players fold to the button, which opens. You raise and your opponent calls your 3bet with a pair of 8's. The flop comes 872 with a heart flush draw (your draw). You bet and he calls the bet. The turn is a 2 that makes a double up on the board. You go all-in on the turn and your opponent turns over his pair of 8's. Here you have 0% equity. Even if you hit your flush, your opponent will still win the hand, a lockout situation.