Basically, in Agar, you are a gelatinous colloid or an agar. The rule is very simple, so-called the jungle law. You need to eat other agars to grow up. The game is ceaseless and will finish when you are eaten by another agar.
The point is a good balance of the game; the bigger you are, the slower you move. That means the game is fun regardless of your size.
You can download the app from Apple store and play it for free. The game may be available from Google play.
Give it a try if you are interested in the world of Agar, but try not to waste your time because you may be addicted, like my son.