Stay is a function that can be given to a minion to make it leave your tribe. When stay has been given to minion they will act like they are from another tribe. Their heath bar will turn the yellow-orange that allied tribes minions have and there is now way currently no way to get them back after doing this.
Stay is currently bugged at the moment. To get it to show up after unlocking it, you need to unlock another skill or technology so that it shows up.