Agar.io is a multiplayer online game where players control a cell in a petri dish. The objective is to grow larger by consuming smaller cells while avoiding being eaten by larger ones. Players can move their cell around the map, collect pellets, and absorb other players to increase their size. The game features a simple interface and can be played in a web browser or on mobile devices. Players can also split their cell to move faster or to capture smaller cells. Agar.io has gained popularity for its competitive gameplay and easy accessibility, attracting millions of players worldwide.