Blender 2.8 Tutorial: Armature Deform with Automatic Weights

Quickly parent your rig and add an automatic weight map to your geometry

Docs:
docs.blender.org/manual/en/latest/animation/arm…

With Automatic Weights
With Automatic Weights parenting works similar to With Empty Groups, but it will not leave the vertex groups empty. It calculates how much influence a particular bone would have on vertices based on the distance from those vertices to a particular bone (“bone heat” algorithm). This influence will be assigned as weights in the vertex groups.

This method of parenting is certainly easier to setup, but it can often lead to armatures which do not deform child objects in ways you would want. Overlaps can occur when it comes to determining which bones should influence certain vertices when calculating influences for more complex armatures and child objects. Symptoms of this confusion are that when transforming the armature in Pose Mode, parts of the child objects do not deform as you expect;If Blender does not give you the results you require, you will have to manually alter the weights of vertices in relation to the vertex groups they belong to and have influence in.