A person's density refers to how many individual strands they have on their head or how much of their scalp is visible when they have their hair in its natural state. If you can see a lot of scalp through your strands, then you have a light density. In contrast, if there is barely any scalp visible, your density is high! In-between is known as medium-density. Thickness refers to the individual thickness of your strands. You can test this by pinching a strand between your fingers. If you can easily feel the strand then your hair is thick, if you can only feel it a little then your hair is considered medium, and if you can't detect it at all then it is considered fine.