Pre-emption simply means that a government entity below the state level cannot pass a law or ordinance more restrictive than state law.
As an example, prior to pre-emption in Texas, state law had a max of 5-1/2" blade length for public carry. Corpus Christi had a 3" limit. State law said nothing about locking folders. San Antonio had made locking folders illegal to carry in public.
After pre-emption was passed, all those local laws were null and void.