That's it.
During the 1.5 minutes of gameplay you are shown a board made out of many colored shapes, and all you need to do is... touch the biggest piece.
Zonr starts off really easy and the more you get right, the higher your score multiplier gets, and the harder the board becomes.
Check out the screen shots below to see a few levels you encounter as you progress:
The big yellow "H" shape is easy to spot here:
Getting harder to spot the largest piece:
Not so easy to quickly find and touch that biggest piece as the levels go up, ah?