Touch the three circles at the same time to gain one point. Repeat until the time is up. Every ten points you earn five extra seconds. Get as far as you can. That's all.
When I've decided that I wouldn't want modifying the game rules, I worked out the appearance and released it on the Android store after two evenings of development time. It was quite painful to get the release working properly since I made the mistake in the first high score setting to mix up the minimum and maximum points allowed in the score table in google play. And you can't change these values after release - which I did at that point...
In the end, I disabled my first release version and created a second version... because in the process I damaged some other stuff that I was unable to repair quickly again.