electrical issue

[HAULN-SS]

So, I put a new alternator in my SS tonight, because I thought that was the problem with it dieing on me a couple of times a while back. Started it at the house, and checked the volt meter int he car, and it was reading normal, took it out, and the whole drive it was reading low, so I didn't want to stop because i wasnt sure if the battery was being drained. So my question is does the volt meter tell me what the alternator is putting out, or the battery? Battery, i guess right? Any suggestions to why the meter in the car is reading low? I had the battery tested at advance, but maybe they dont know what the shit they are talking about