Unblocking a suit in a trump contract
This occurs when one (ore more) honor is blocking a secondary suit of yours.
To solve the problem and cash tricks in this suit, you need to unlash it. Therefore, when you play your leading trumps, discard the embarrassing high honors.
Example
You are South playing 4♥. You need 4 ♦ tricks.
Trump is your only option to be in hand.
How can you make 4 ♦ tricks without being ruffed ?
West leads Ace ♣.