Charm is a Skill in Bannerlord.
It is the ability to make a person like and trust you. You can make a good guess at people's motivations and the kinds of arguments to which they'll respond.
With the participation of the ancient ritual of exchanging one coin for another, one's charm can be increased. The ritual is very intricate as it often involves the exchanging of spirits, foods, and stories over a fire. Naturally, this socialization is a solid way to increase one's charm.