Definition of blocker
A "blocker" is, you guessed it, a card that blocks. Nothing better than a good example to clarify. You are on the river with an ace of hearts and a deuce 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 bets before you. The board reads: 4-7-9-J-K with 3 hearts. Your opponent could have a flush. He is a very passive type and gives you a lot of credit. The best possible hand on this board is an ace. You don't have the flush, but you know 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 gained is a plus. Going back to the example above, knowing that your opponent doesn't have the ace flush allows us to represent that flush and bluff more successfully. If he's a conservative type, there's a good chance he'll fold against our all in. Here, we will say that we bluffed because we had a blocker at the nuts.