Komi is a way of settling the first player's advantage. Many high level games would be needed to establish a reliable number. Nobody ever objected to three though. Indicating komi is not the marker's primary function - it is there because refraining from putting a stone on the board at one's turn earns a point, indicated by moving the marker. Although the track has 19 points, its unusual for it to move more than a few points either way.
Players move in turn.
Black moves first. A move consists of placing a stone on a vacant point. If a player passes his turn, he moves the marker one point towards him. The player at whose side the marker is at the end of the game, receives the corresponding number of points in addition to the points he has on the board. A group consists of a single stone or several connected stones of the same colour.