Paper.io 2 is a popular online game by Voodoo.
The gameplay is simple but hellishly addictive. You control a square paper that leaves a colored trail as you move across a blank grid. Your goal is to grow your territory by encasing areas with your colored trail. The catch is other players are doing the same and you must not crash into their trails.
To grow your territory you need to draw closed shapes by circling them with your paper’s tail. Once you close a shape the area inside becomes your territory and it’s filled with your paper color.