Definition of lockout
A lockout is when a card helps a player by making it impossible for him to lose. In other words, when one card makes one player lockout0% fair and the other is drawing dead, it is said to be a lockout (you could say it is now in the pocket). When we say in poker that we're drawing dead, it means that our equity is 0. When you're drawing dead, it means you're all in with a draw, but even if you hit your draw, you still won't have the best hand.
An example of lockout
You are on the small blind with AK in the heart. The players lie down until the button that opens. You raise and your opponent calls your 3bet with a pair of 8's. The flop comes 872 with a heart flush (your flush). You bet and he calls the bet. The turn is a 2 which makes a double on the board. You go all in on the turn and your opponent flips his pair of 8's. Here you have 0% equity. Even if you hit your flush, your opponent will still win the hand, it's a lockout situation (also called a lock, a sure win).