BlockerBlockers are cards that a player needs to complete his draw but that are already in an opponent's hand.
Example (Texas Hold'em):
Both players have inside straight draws. Player A needs a seven and B a six to make the straight 45678. Hence, they each hold a card that the other needs; A has B's six and B has A's seven. These cards are blockers. There are no longer 4 sixes and 4 sevens available for completing the draws, but just 3. This reduces the chances that either player successfully completes the straight.