Armature Modifier
The Armature modifier tells the mesh how it will be deformed by the bones of an armature. As with constraints, there are two ways of adding an Armature modifier to your mesh:
Select the mesh, go to the Modifiers tab in the Properties Editor and add an Armature modifier. Then, in the Armature field, input the name of the armature you want to deform the model. In this case, it should be Jim_rig.
Another way to add an Armature modifier is to select the mesh, then select a bone or armature while pressing Shift, and from there, press Ctrl + P to parent. When you parent a mesh to an armature, Blender will show you several options, some of which are Armature options. Pick the option for Automatic Weights; it’s usually the easiest option (unless you have set up envelopes for the bones, which is another method that won’t be covered in this book).
Caution
If you have previously added a Subdivide Surface modifier to your models, when you add an Armature modifier it will be working on top of the SubSurf, which will reduce performance and make it more difficult to weight the vertices. Move the modifier up through the modifiers stack until it’s placed before the SubSurf modifier (SubSurf should be the last modifier, at the bottom of the stack).