This is a popular game similar to all Tetris clones, this time with colors to gather instead of lines.
A three-tile column falls from the top of the screen. You can use the keyboard arrows (or your joystick/joypad) to move the column, and the up arrow to swap the three tiles between themselves.
The purpose is to form columns, rows or diagonals of at least three tiles of the same color. The more tiles are in a "column", the more points you earn. When you form a column, its tiles disappear and the other tiles can fall.
If they form another column when falling (it's usually called a "chain reaction"), you get even more points. Basically, the points are doubled when the column is in a first chain reaction. When a column is formed in a second chain reaction, the points are tripled, etc.
Every time you remove 30 tiles from the board, the speed rises a bit.