Digital rigs are the virtual bones, joints, and muscles that allow models to move. It’s kind of like the strings on a marionette. A good rig has just the right amount of flexibility. Without the right controls, the animators can’t create the poses they need. Too much flexibility makes posing the model too time consuming.
Riggers start with a virtual 3D model for a character. They study how a character needs to move based on the story. For example, Randall in Monsters, Inc. moves like a chameleon, but he also walks on two legs. Riggers break down those motions into individual elements and create the hundreds of control points that animators will use to create poses.