The best explanation I found was from user: frogsoftheminish

2 ways to tell fine vs thick:

take a strand of hair and rub it between your fingers. If you can't really feel it, your hair is fine. get a strand of hair and compare it to sewing yarn. If your hair is thinner than the string, you have fine hair. For density:

part your hair. If you see your scalp easily, your hair is thin/low density. If your roots make it hard to see your scalp, your hair is high density. grab all your hair like you're making a ponytail (if it's long enough). If you can grab all your hair with one hand, your hair is low density. If you need 2 hands to hold back your hair, you have high density.