There are too many articles written on prophylaxis thinking in chess but how it can be explained in simplest terminology? The simple answer which a lay man can understand is ‘Prevention is better than cure’. I hope you understand the concept.
You are not allowing your opponent to create threat against you or you are not allowing your opponent to place their piece in decisive square or position or even you are stopping your opponent to improve his/her position called prophylaxis.
For example you are playing h3 to avoid back rank mate or to avoid pin, these moves are prophylaxis which we are playing subconsciously without having knowledge of prophylaxis thinking. Prophylaxis doesn't mean only pawn move. According to Mark Dvoretsky ‘’Prophylactic thinking is the habit of constantly asking yourself what your opponent wants to do, where he would go on his move, the ability to find a reply to the question that has been posed and to take it into account in your decision-making process’’
Now when you understand what prophylaxis thinking is, the major thing you need to do is, implementing the same in our game. As we all know planning is not a hard task to improve your chess but execution/implementation is the problem.