When you request to join a 👑 liege, you are asking permission to join someone's kingdom 🏰.
Upon being accepted, you will enter into vassalage of that player and they will become your liege. Your 👑 liege can send you vassal gifts 🎁 and you may accept 1 every 24 hours.
As a vassal of your 👑 liege, it is their duty to protect you from being seized from their kingdom. Without a 👑 liege your kingdom could be conquered.