Influence, something or someone that puts a spell on you, etc. Jim Dixon is correct. A charm can be a person as well as an object.
One still hears "He is a charm" in the South today. My wife uses it.
"He is a love," or "He captivates me" express some of the same thought.

Many different usages of the old word; the first cited in the Oxford is "a chanting or recitation of a verse...with magic power" to ward off evil or to cast a spell. 'Charm' also means a songbird, etc. etc.