What is Prophylaxis ?

Prophylaxis : 
If we can prevent our opponent's ideas and also improve our own position with one single move , then such kind of move is called a prophylactic move.
There is a big difference between defensive moves and prophylactic moves. Prophylactic move also improves our own position but defensive move doesn't. Remember -"Defensive moves can be ugly , but prophylactic moves are always beautiful.''

I will show some examples so that you guys can understand what prophylaxis is all about !

It's white to move. It is obvious that white's bishop on c5 is extremely strong.  It is also preventing black from castling. So black's main idea in this position is to play Nd8 followed by Nb7 to remove white's active bishop from c5. So, Can you find a move that improves white's position and also prevents black from accomplishing his idea? 
The answer is .... the amazing move Bd1!!

 The explanations of Bd1 is given in the analysis.

Let's move on to the next example.

It's white to move. Black wants to play Bxf3 followed by dxc4 and e5 , and the other plan is dxc4 followed by Bxf3 and e5. Can you find a move that prevents those ideas and also improves white's position? 

The answer is really simple. Just a casual move Rfd1! The beauty of chess is you could even play Rfd1 here without even realizing its true power. Because it is so natural. You may ask , how Rfd1 prevents black's ideas !?  Well, check the analysis.

Here's another example. Hope you are not tired lol. 

It's white to move. Black wants to play a4 and trap the bishop. How would you prevent that and at the same time improve your own position?

The answer is Rh4!