This should be in off-topic.
I don't even want to Google agar.io.
Probably should be, but it's not inappropriate. It's a game in which you're a blob and you eat smaller blobs and become a bigger blob. You have the option to split yourself in half which shoots half of yourself out in the direction you split, and you can also shoot small blobs out of you using W into the bigger spiky balls on the map to make more of them.
If you try to eat one of those spiky balls you're forcibly split like crazy, a good strategy is to walk up to the rank 1 and shoot one at them and then eat away. There's no winning. If you stop eating you deteriorate. As you get bigger your rate of deterioration goes up. So it's fun to just get rank 1 and then try to trap people and see how long you stay rank 1.
Also if you name yourself like USA, you'll be given an American flag as a skin. It works for most countries. There's also a lot of words that will give you a skin, someone else could probably find a list.