Go to the following link and you'll find a pretty good explanation:

http://www.chuckhawks.com/bc.htm

As far as how it relates to hunting, well, let's just say that a high BC bullet is really not required unless you're shooting things way out there where bullet drop becomes a real issue. At the ranges most people shoot big game, 150 yards or less, the ballistic coefficient of the bullet really doesn't matter much at all.