Khan Academy actually has a free online program in partnership with Pixar that introduces many of the techniques used by their animators. They have a whole submodule dedicated to subdivision and its benefits in animation (https://www.khanacademy.org/partner-content/pixar/modeling-character#modeling-subdivision). This could be useful to check out both to understand subdivision better conceptually, and to practice its application.
I looked a bit into the Khan Academy ^ and it explained that subdivision helps the character's surface look smooth and realistic. With two simple steps--split and average--you can turn a square into a head!
The Khan Academy video really explained a lot. Subdivision is really interested. I remember watching Geri's Game years ago and never really appreciated the detail until re-watching it today.