Refers to the amount of time added to each player's time before each move. For instance, rapid chess might be played with "25 minutes plus 10 second per move increment", meaning that each player starts with 25 minutes on their clock, and this increments by 10 seconds after (or before) each move, usually using the Fischer Delay method. See Time control#Compensation (delay methods).

It's an amount of time added to the clock after each move is made. For example in a 15/10 game, one starts with 15 minutes and get 10 seconds added to his/her clock after each move.