Agar.io is the Android version of the game that can be accessed at that specific web address. Your entire purpose is to move a tiny sphere around a plain and to absord smaller dots. The catch is that you will play against other players and if their spheres are bigger than yours, you can easily get killed and you will have to start over again.
If you want to play having the flag of your country, you should pick the country name as your username. Many other secret skins can be used as long as you know the proper username that must be typed. When it comes to its control system, you can move this sphere around the plain by touching the touchscreen of your device with the finger.
Agar.io is a very minimalistic mobile game making it easy to handle even by older devices that aren't that powerful with regard to hardware configurations.