You are referring to Live Chess, correct?
It's a time control setting. If you set up a game with time parameters of, say,"15 10", that would mean 15 minutes, plus 10 seconds extra per move. If the parameters were "15 0", the clock would just run for a straight 15 minutes per side.
It's a nice option that digital timers make possible.
Hope this helps answer your question!
I noticed that while playing online chess oponent timing is increasing 5 to 10 seconds for each and every move. How it happent? can you help me anybody. Thank you.