Let the blue ball move in the field without touching the red balls.
Click on the empty field to start a game: a blue ball immediately replaces your mouse cursor, and a number of red balls start to fade into the field.
When the red balls finish materializing, they move at a random speed in a random direction, and you have to promptly dodge them moving the blue ball inside the field with your mouse. After some time, other red balls will appear, fading in as before and then starting to move. Be prepared to avoid them, too.
You can choose the game difficulty level on the right of the status bar. The higher the difficulty level, the faster red balls move, and the more balls will appear on the play field.