Pinning is a term for a chess move that many chess players already know how to execute. If you do not know this move it is vital that you practice it, learn it, and look for opportunities in games to execute it. This is a fundamental tactic to use in games. It is known by different terms including pinning, double threat, and the pin.
Pinning is being able to threaten two pieces at the same time with your one piece. This forces your opponent to decide which piece to sacrifice. An example of this is placing a bishop in diagonal file that threatens to take two pieces. Whichever piece your opponent moves, you will be able to take the other. Another example would be placing a rook in an open file so that it threatens both a knight and a bishop. If your opponent moves the knight, you can take the bishop, if your opponent moves the bishop then you can take the knight. A knight is a wonderful piece to use in creating a pin because of its versatility and due to the fact that some inexperienced players simply overlook the available moves that your knight has in front of it.
Defending against the pin consists of two strategies. First is the option of retreating so that both of your pieces are defended by one another. This is not always an available option though. The second defense is to threaten a highly valued piece that your opponent has, generally a queen. If you are being threatened with a pin and can threaten your opponent's queen, your opponent is likely to defend that queen before taking advantage of the pin they have laid out.