That's "Chemistry, the Active Skill" as opposed to "Chemistry, the Knowledge Skill." Granted, I'm not sure how you would have the first without the second, but I can easily see having the second without the first. Knowledge means you have the basic understanding of chemistry (covalent bonds and all the other stuff I forgot from HS Chemistry). Active involves actually being able to work around a lab, understanding safety procedures, not blowing yourself up or creating toxic clouds on the reg, etc..
In older editions, you received free knowledge skill ranks equal to half your active skill ranks in the appropriate areas.
