Agar.io is a popular multiplayer online game where players control a small cell and must grow by eating smaller cells while avoiding being eaten by larger ones. Players can split their cell into multiple smaller cells to increase their chances of survival, but risk becoming more vulnerable. The game features a simple yet addictive gameplay mechanics, smooth controls, and fast-paced action. Players can customize their cells with different skins and compete against others in various game modes. Agar.io offers a competitive and challenging experience that requires strategy, skill, and quick reflexes to succeed in the ever-changing environment.