What does boiling do and why is not legal?

OK Guys,
Rookie question,what happens when you boil a base?If it makes it faster how does it make it faster?Why is not legal to use a boiled bases?I think I read somewhere that not everyone knows how to boil a base.Can you tell by the appearence of the base it has been boiled?Does boiling disfigure the base?
Thanks Ive been curious about this for a while.
Erick