This game is called Agar.io. It has become extremely famous and all over the internet. Many youtubers like Vikkstar123 have been playing this. Thre are three modes: FFA, Teams and Experimental.
The point of the game is that u eat cells that are smaller than u. The cells in the picture don't move at all and are harmless so i advise u to eat those before eting other people; tis helps u build up. People bigger than u would obviously try to eat u- in thiss case, u must be careful and run away quickly. The smaller u are, the quicker u run.
The main instructions are:
Move ur mouse to control ur cell
Press space to split
Press W to enject some mass (useful when ur on the mode Teams)
It doesn't matter if u forget these instructions as it says it under PLAY.
(In the mode Team there are three teams: red, green and blue. They choose ur team for u).