FFA Mode (which stands for Free-For-All) is one of the 4 currently available modes of the game. In this mode you are on your own against all the other players in the lobby. Even though some are forming teams due to their lack of skills.
Your objective is to grow and to become number 1 on the leaderboard (and to try to stay there). The places on the leaderboard are determined by total mass of the player's cells. You can consume any other player in the lobby.
In this mode you can use Skins although you will still have a colored outline around it.
Occasionally, a server will stop re-spawning cells and instead spawn new cells in another server. This leads to a last-man-standing style of deathmatch among the remaining cells, as any cell that is eaten will leave the server (Pretty much elimination here) . This deathmatch will last roughly 10-20 minutes, at the end of which all cells will be transferred to another server, alive or not. Generally, only cells in the top 10 will remain alive to compete in the deathmatch.