At first glance, this is a primitive browser-based multiplayer arcade, migrated to mobile platforms. You are the ball that eats other balls and grows. While you are small, you are prey, and by gaining mass, you become a hunter. But the beauty of agar.io is in the details:
Small balls are very fast, while more are slow.
Strongly growing on eating “feed” – scattered around the location of the pellets – will not work, you need to eat other players.
To catch up with a smaller quick ball, you can split in two or shoot at it with a small part of your mass. But there is a risk that someone who eats more is eating you.
If you fell apart into small balls, you are going for a long time. At such moments you are vulnerable.
When you see that you are on the sight of a large ball and it is approaching, it is worth sharing – there is a chance to escape.
“Spines” break large balls into many smaller ones, but they are not dangerous for babies. You can hide behind them.