After you have entered the paint weight mode, you will see the whole character turn blue with a small section of red on its body. These colors represent the skin weight values of the vertices associated with a particular bone. You can try to select a different vertex group and see the colors changing. This is because you're now viewing the skin weight values associated with another bone.

To change the value of the skin weights, we can paint the values using our mouse cursor. To change the size of the brush, go to the Tool Shelf and you will see the radius property under the Brush section. There are also some other options that you can set, such as the types of brushes, weight value, strength of the brush, blending mode, and so on and so forth.

There are also some advanced settings below it, but we will skip those for now.

To make the skin more fluid and less robotic when animating, make sure that each bone has some minor influence over the neighboring vertices beside the ones...