A blockade is a situation when the opponent has a pawn that needs to be stopped. Usually it is a passed pawn and it will promote unless stopped. A easy way to stop a passed pawn is to put a piece in front of it. That way, it can't advance. The best piece to do this with is a knight. Queens and rooks shouldn't be used unless you have to since they shouldn't be tied down doing this. You can block pawns with pawns too. So usually if your pawn gets blockaded, you should remove the blockader. Here are fun examples.
But one thing you should remember is knights can not hold a passed pawn back from behind. When the king attacks, the knight has to go to the side. Knights also can't defend rook pawns, since they are near the edge, and knights are dim there. Hope you liked it,feel free to comment.