What does that mean?

I received a call from my doctor's office this afternoon, informing me that I have low carbon dioxide levels in my blood. I didn't even know this was something that was measured. A quick search of the internet of course revealed a variety of horrible things this could mean, but, in general, suggested it wasn't too much to worry about. I am going to have the follow up test, though--might as well take advantage of the fact that I have health insurance.