Blocking is an action only available to creatures with the Blocker ability. Whenever your opponent's creature attacks, you may tap the blocker. The attack is stopped, and then both the blocking and the attacking creature battle as if the attacking creature is attacking the blocker. Effects that trigger when a creature attacks happen before anyone can choose to block the attack.
Blocking is an important part of game-play as it can prevent your opponent from getting at your shields too quickly, or from taking out vulnerable creatures after they tap to attack. Only untapped blockers are able to block attacks.