A suit has a blockage if it is impossible, without the use of an outside entry, to play out the suit by cashing top cards. If you block a suit, it means you play a suit in such a way as to create a blockage (not normally a good idea). For example:
Layout 1 starts as blocked. To cash four tricks, South must possess an outside entry since the ace will win the first round in the North hand. On Layout 2, you can avoid a blockage by cashing the ace first; if, instead, you cashed one of the honours from the longer holding, then that would block the suit.