You are a blob, wandering around and absorbing smaller blobs to grow.
Larger blobs, likewise, seek to absorb you. It's a familiar mechanism (cf. Katamari Damacy, Osmos, fl0w) but agar.io strips it to the bare cell and pits you against others online.
Tips: hit space to divide, avoid the edges of blobspace, and use the spiky green things to slow pursuers.