Definition of blocker
A "blocker" is, as you may have guessed, a blocking card. Nothing better than a good example to clarify. You are on the river with an ace of hearts and a two of spades. The important card here will be the ace of hearts which will be our blocker. You are in position on the river and your opponent is ahead of you. The table reads: 4-7-9-J-K with 3 hearts. Your opponent may have a suit. He is the very passive type and gives you a lot of credit. The best possible hand on this board is an ace flush. You don't have the flush, but you know that your opponent doesn't have it because you block it with your ace.
Usefulness of a blocker
A blocker should be seen as information. Poker is a game of incomplete information and every piece of information obtained is a plus. To return to the example presented above, knowing that your opponent does not have the ace flush allows us to represent that flush and bluff more successfully. If he is the conservative type, chances are he will fold against our all in. Here we will say that we bluffed because we had a blocker to the nuts.