The sacrifice Altar can be used to sacrifice a unit for recourses.
Sacrifice Altar is a building where you put your unit in there, and in return get something like food or gold.
The Sacrifice Altar is useful to get rid of weaker troops that are no longer needed, in return for an amount of resources. However, the amount given depends on the troop. Example: A swordsman might give you 50-60 food.
Any unit Sacrificed at this altar can't be returned, not even by the graveyard. When you have a pop up, you might accidently send a unit into the Sacrifice Altar so it's recommended to store it if you don't need it!