ok, layman terms.
muscle is made out of protein.
you lift weights and 'damage' the proteins.
you eat food, including protein.
some of this protein will go towards various functions - one being the repair and subsequent growth of the 'damaged' muscle.
The repair in essence, makes the muscle more adaptable to the weight of the exercise thus, continual optimal growth will only come from lifting heavier next time or with more repetitions.
if you 'damage' the muscle too much and/or too frequently then repair will not occur quick enough to allow this unless you're on gear. The body can only repair/grow optimally in most natural lifters with lower volume and more rest.
that pretty much sums it up. can anyone spot anything i missed?