One of my kids got me hooked on this. It's incredibly simple but also kind of fun and addictive.
You start as a small cell and need to eat things to grow. That's pretty much the game.
Of course as you get bigger you can eat smaller cells too ... and bigger ones can eat you.
As you get bigger you get slower but you can "eject mass" to shrink and speed you up (w key). You can also split to devour a smaller cell and will eventually re-merge but in the meantime you are more vulnerable.
You can eventually get super-massive and as you get larger the tactics of the game change. People form alliances (send some mass as a token to feed someone else) or become intent on hunting you down. The viruses (green starts) protect you when you are small but will fragment a larger cell into many tiny pieces (feeding frenzy!)
Play in your browser or on a mobile. Way easier on a desktop with a mouse though.