In this two player game, you must be the first to create an line of four marbles of your color. Each player drop a marble, turn by turn, by clicking at the base of the target column.
Indeed, the marbles can be stacked; thus, the game grid is four by four by four marbles. A successful line can be oriented diagonally, and can even span from a corner at the bottom, to an opposite corner at the top.
An AI can be enabled by pressing O; however, it is very basic, as it will only predict a couple of turns in advance, and very slow.
If you want to play a new game, just press R.