Robin Hood could probably charm his way through an entire adventure, just by smiling at the people he meets and carousing his way. Some people are naturally charming, and can make people swoon; very handy if you need to get people to do something for you, or to let you into a facility. The Charming trait reflects this additional seductive quality, and gives them an edge when trying to schmooze their way through any situation.
Charming is a Minor Good Trait, and when the character is trying to charm their way through an encounter, they receive a +2 bonus to the roll. Useful for talking your way out of being killed, but not always suitable for every social situation.