Empty the field as quick as possible of all the blocks before the time is up and they fall down. If they reach the bottom you are game over. You remove the blocks from the field by making lines in matching colours and letting them hit the right blocks. Do it fast and in the right order, because the patterns get more and more complex.
Use the mouse to play. Use the number keys to select a colour (or click on it) and drag the mouse horizontally across the field to create a line in the selected colour.