Up until now, you have only dealt with static 3D objects. Maybe you have a daring piece of update loop code that transforms models so they move around like chess pieces on a board. But if the moving objects are characters, you would like to see their legs and arms move while they walk. It's time to animate your 3D models.
In 3D games, a method called skeletal animation is used to animate characters. But in principle, the skeleton approach can be extended to any 3D mesh – even an opening crate's hinge can be considered a simple joint.
Tip
Your wizard fidgets, your ninja moonwalks, and your brawny warrior skips along like a balloon animal? Unless you are animating toons, the realism of an animated character's motion is generally a problem. Professional game designers invest a lot of effort to make characters animate in a natural way, including the use of motion capturing.