Four in a Row is a two-player game.
Taking turn, each player drops a coloured piece to one of the 7-columns of the board. The piece will then travel down to the next empty row in that column.
The first player to form a connection of four pieces horizontally, vertically, or diagonally wins the game. If the board is completely filled but no one has won, the game is a draw.
For a more detailed explanation, you can refer to the Wikipedia article on this game.
Four in a Row