I've always been really into cars and recently found this site. I am curious as to what exactly a "built motor" means. Thanks in advance

just that built motor.

From the main page...
Deals with anything regarding "Building, Modifying, or Swapping Engine Internal Components".
(if you don't get this one, you're definitely a bolt-on-only type of guy.)

The generally accepted definition around here is internal motor work and so on. i.e aftermarket pistons, rods, head-work, high performance short blocks. ect.

thanks a lot guys