HCG is short for Human chorionic gonadotropin which is a specific hormone that is released into your body (blood, urine) at the time of pregnancy, it helps sustain an early pregnancy until the placenta can take over, this is what your hpt picks up on to give you a positive, there are two types of blood te4sts that measure this, one just says yes or no to pregnancy, the other is a couple of tests 2-3 days apart and this measure the amunt the hcg rises. for a pregnancy to considered viable usually the hcg should double or at the mimum rise by 60% during this time frame, there are some variables though, but usually once you see levels starting to fall, a miscarriage is on its way
I'm 36 weeks and I've never known what mine is. I wouldn't worry about it unless there's some reason you need to check them, some indication that things aren't developing right. There are enough things to obsess over in pregnancy already. :)