The block is a stroke played in order to control an attacking shot. You would most commonly block against a loop, simply volleying the ball back safely.

An aggressive block would be a block with more force pushed back in order to produce a faster return. This is often performed against a player who attacks but is in a bad location (off to the side), leaving a wide area open for the receiver to aggressively block the ball back into the open court.

See the Block stroke section in the Coach Wiki.