A blocker card is one that you hold and block to a potential premium hand.
For example, AK can be played aggressively against a lot of opponents since you have a blocker to AA/KK.
For the same reason, 3-betting light with blockers in your hand is a good strategy against the right kind of player. To better understand this, watch the video below.
Blockers are useful in trying to figure out what an opponent has since you can limit some of the potential hands in his/her range.
EXAMPLE “With the nut flush blocker, I decided to represent the flush on the turn.”