Free-For-All, colloquially termed "FFA", is the exact opposite of Team-based gametypes.
In Free-For-All, every player in the match is an enemy. Players no longer work in teams and must be dependant enough to survive and eliminate other players. Performing suicide will extend the respawn time and one point will be deducted from the suicider's overall score.
Halo 5: Guardians
"Free-for-All" is the main playlist for Slayer FFA in Halo 5: Guardians' Arena playlists. It originally had 8 players, but 343 Industries decided to make it more competitive by diminishing the number to 6 players. This was done in December 2015, with the Cartographer's Gift update.