Im going to take a shot at it, although im no expert but when you workout you tear down your muscles and when they rebuild they bind together stronger and that increases the muscle fibers and the diameter or the muscle?
in really really really simple terms it works SOMEWHAT like this...
when you get a cut, what happens? your skin heals at the spot of injury and creates a scar, which is many times stronger and more durable than the original skin.
in a way, rebuilding stronger muscles is your body's way of insuring that the breakdown and destruction of muscle fibers (lifting) wont happen again.
again this is a very very basic and simple analogy.
It is the process in which you break muscle fiber by overload(ie: weight lifting) and after they recover and grow a little bit bigger each time.....
Hypertrophy only make the mucle fibers bigger and does not increase the number of fiber only is size change.....You have to take HGH if you want more muscle fibers....