Prophylaxis is the chess term for stopping an opponent's plan or threat before it has been carried out. Discussion of prophylaxis often makes it sound like a grandmasterly concept that us mere mortals can't figure out. However, all that is required is being alert and calculating variations with an open mind throughout a game. You just need to realize that you're opponent gets to make moves too that are often just as important as yours!
One good example came from a World Championship match between Max Euwe and Alexander Alekhine. In the following diagram black will be fine if he gets to unpin his bishop with Bb5. How did World Champion Euwe stop it from happening?
Euwe played Ra2! Black's only move to get out of the pin is Bb5, but now white can just capture it with cxb5 and be up a piece. Black was paralyzed and white went on to win quickly.
I try to use these techniques in my own games and when I'm playing well, prophylaxis often plays a big part. Here's a game of mine where I had to stop black's plans throughout the game. Despite black's gambit, he was never able to build an initative and I my position slowly improved, leading to a smooth win. Enjoy.
Hopefully, you enjoyed the game and maybe learned something. If you would like to practice preventing the opponent's tactics, see if you can do better than GM Donner did in the following diagram. Black has sacrificed a piece against GM Miguel Najdorf, but white has just play c5 - c6 with the hope of keeping his material. In the game, this trick worked. What should black play? Feel free to post an answer in the comments.
Thanks for reading!