It's often used to describe a person (especially a woman) who is plump, full-bodied, and considered attractive in a sensual way. It derives from the Flemish painter Peter Paul Rubens, who was known for depicting women with this type of physique.
While it can sometimes be used to describe someone as being overweight, it's not always a negative term. It's important to be sensitive to the context and the person's feelings when using this word.