It means if your housing unit is already regulated under Costa Hawkins, a city cannot pass stricter laws on it. Meaning since Costa Hawkins protects SFH/townhomes/condos, a city cannot pass stricter laws against them.
It also means that if a city does not have preexisting rules on MFH that are more strict than AB1482 (like SF does), then after the implementation of AB1482, a city cannot make new rules against that MFH that are stricter than AB1482.