In the game of chess a blockade is a wonderful tactical method that forces your opponent to move his or her pieces into the path of their other primary valuable pieces thus blocking those primary pieces from being able to attack you. Think about in the opening game when you are able to possibly threaten a bishop that is unguarded. This results in your opponent moving that bishop in front of his or her own queen. Once you have done this you have successfully carried out a blockade on your opponent. Now the queen cannot move to attack you, nor can it serve as a defender since the bishop that you had threatened just one move ago blocks it in.
There are many other examples of ways to use the blockade. One of the more effective methods is used in the end game. This is where you are able to threaten the king and force your opponents king to move into such a position that your opponents other pieces are no longer a threat to you. Place your opponents king into check and that king is forced to move. If you are able to limit where the king can move, so that the king moves in front of one of his primary pieces then you have again successfully carried out a blockade.
This is an important tactical skill for beginners to learn and to watch for. More experienced players are often wise to this tactic and will be watching for it so it may not be quite as successful against those more experienced players. Regardless of your particular skill level this is a fundamental tactical skill that must be used on occasion and a tactical skill that needs to be actively guarded against.