Cats headbutt, or head rub you as a sign of affection… and also as a kind of sign of ownership, a way to keep track of you.

Cats produces pheremones from their foreheads, and rubbing said pheremones on you is basically a way of marking territory.

If another cat has been rubbing its forehead on you, and you come home to your cat… your cat basically knows you cheated on it, lol.

Or, maybe its hapoy you made a new friend.