It means that out of the required courses needed to enter their dental school, only up to 30% of them can be taken at a CC.
Bio 1
Bio 2
Physics 1
Physics 2
Gen Chem 1
Gen Chem 2
Organic Chem 1
Organic Chem 2
Biochem
So in regards to the above list, only 2 or 3 of these can be taken at a CC. If you're a biology major though and have lots of upper level science courses at a university like micro, cell bio, genetics, anatomy & physiology, etc then I wouldn't worry about Pitt's limitations on CC credits.
Basically don't take more than two or three sciences at a CC. If it tells you anything I took Bio 1 and 2 while I was still in high school at a CC, and took Physics 1 and 2 at a CC in a summer at a CC. I'm still applying to Pitt because all my other sciences were from a four year university (Gen Chem 1, Gen Chem 2, Organic 1, Organic 2, Biochem, Anatomy 1, Anatomy 2). They just don't want you taking half or more than half of your sciences at a CC. If I had to give you advice, I'd advise to knock out physics at a CC and do the rest of your sciences at a four year university.