Something that's jointed has separate sections that are joined together. A jointed doll has arms that bend at the shoulders and elbows and legs that move at the hips and knees.
A stuffed animal that's jointed has distinct points where its legs meet its body, and these joints usually make the legs movable. You are jointed, with segments of your body connecting at joints (like your wrists, elbows, and so on), and if you're "double jointed," these joints are extra flexible. The Latin root of jointed and joint is iunctus, "united or connected," from iungere, "join."