Your question's answer is written in Go rules : Since the black player starts the game, he has a significant advantage. In order to balance it, a value called komi is subtracted from the black player score. It is 5.5 for a 19x19 board, 4.5 for a 13x13 board and 3.5 for a 9x9 board.
(hide) Play a game in real time with an online opponent! For this to be possible, you and your opponent have to select the action "Move and stay here" as default and then reload the page with the F5 key! (TeamBundy) (show all tips)