A joint is a transform in a digital skeleton that helps define a pose. Moving or rotating joints can carry connected parts and, through skinning, deform a character’s surface.
An elbow is a useful starting point
Picture an articulated wooden figure. Turn its elbow and the forearm and hand follow, while the upper arm can stay still. A digital joint serves a similar purpose, although it is a transform rather than a physical hinge. The animation highlights a stationary elbow and the changing position of its child. This is a rigid chain: there is no soft surface being bent in this example.
Joint, bone and skin are different parts of the explanation
Applications use “bone” and “joint” in different ways. A line between joint markers often visualises a skeletal relationship; it is not necessarily visible geometry in the finished artwork. In glTF, a skin lists the nodes used as its joints. Rigging establishes the skeleton and controls. Skinning connects mesh vertices to joints with weights, allowing a surface to follow their motion. Moving a skeleton will not deform an unrelated mesh that has never been bound to it.
Why the hierarchy matters
Joints are commonly arranged in parent-child chains. A hand can inherit motion from an elbow, which inherits motion from a shoulder. Its local rotation and its final world orientation therefore describe different things. A joint also has a reference pose used by the skinning setup; casually changing that relationship can produce unexpected deformation. Not every animation needs a skeleton: a floating caption or a spinning sculpture can be animated as an ordinary object.
From a character file to an AR artwork
When preparing a character for Painta, inspect the exported GLB as well as the source project. Check a few difficult poses, such as a sharply bent elbow or a raised shoulder. If the whole forearm follows the wrong point, inspect the joint hierarchy and transforms. If the chain moves correctly but the surface pinches, inspect the skin weights. These checks separate the motion of the skeleton from the behaviour of the visible mesh.