15/10

Please explain again what that means.

Never mind. I got it. I don't understand why add 10 seconds for each move. What's the point?

could you explain it to me then? i have never understood

In OTB paly, it gives you time to write down the move.  When you are playing online, your moves are automatically written down. I don't know why they have it either

It's so when you get in time trouble, the opponent is forced to actually try and mate you since he can't easily run you out of time.  I've heard commentators call it "living on the increment".

15/10 means you start with 15 minutes on your clock. Each time you make a move you get 10 seconds added to your clock.

The idea is that you always will have at least 10 seconds to make a move even when in serious time trouble.

Well when I started playing chess I got a Chess Clock. Then in the 80's it just had normal analog funktions and if you wanted to play Blitz Chess you syncronized 5 min for both players. Nowdays they have digital Chess Clocks so they can play a blitz game with the FIDE rules 5 min or 3 min with 2 seconds adding for each move. I guess the meaning with that is to minimize the risk that someone loosing on time because he can't mate the oponent though he is the much better player. Then when you play a full length game with FIDE rules you will also add time for each move in the endgame. That is good so you get time to write down the moves.