There are two things that any chess player should consider during a chess game. First, have a game plan and make sure your moves have a purpose that will help execute your game plan. Second, stop your opponent from executing their game plan. Prophylaxis is a weapon for this second point.
Prophylaxis is a chess tactic where one side tries to prevent their opponent from moving to a critical square or executing their game plan. While these moves are usually seen as purely defensive, they can improve one’s position and open up options that would not be available if a prophylactic move wasn’t made.
One of the most common examples of this tactic can be seen in the Najdorf Defense. On the 5th move, black plays a6. This stops both of white’s knights from coming to b5 and also stops the light squared bishop from coming to b5 and checking the king. While a6 may not seem like a great move, the Najdorf Defense is one of the most popular defenses in the Sicilian Defense and shows the power of prophylaxis.
To watch an overview of prophylaxis and see examples you can check out the video below.