From Halopedia, the Halo wiki
Valor can be used to unlock legendary weapons, vehicles, and Marine deployments at controlled FOBs. It is awarded for completing side objectives, such as taking out Banished Outposts, destroying Propaganda towers, and rescuing trapped or captured squads of Marines. It is also awarded upon the completion of main campaign missions. Valor will still be awarded for completing a rescue mission, even if all the Marines are killed.
For the list of items unlocked in the FOB progression system by gathering Valor, see here.