Some encounters include monsters or NPCs that cooperate with the party rather than act as adversaries. You can mark these monsters as friendly in the combat tracker.
After you set a monster as a friend they behave slightly different:
Huddle on the map with other characters
Count as characters when computing encounter difficulty since they will likely help the players.
Not contribute experience points to the encounter.
Hint: if the monster is going to travel with the characters click the keep option when finishing the encounter so that they won’t be removed from the combat tracker.