Friendly Fire is a term that is used to describe the act of a teammate causing damage(reducing the hit-points) to a unit on the same team. Friendly Fire is usually accidental due to it being counter-productive to winning a game, but it can be deliberate. Games can limit or prevent friendly fire all together. In Warhawk, friendly fire was initially on for default settings in official games but was turned off in later updates. It was still possible to damage and kill teammates after the update, but was far more limited and very situational.
The specifics of friendly fire are unknown for Starhawk, but it will very likely be a selectable option to turn on or off. The default settings on friendly fire for ranked games is also not known.