Hair density is one of the markers we cosmetologists use to assess hair. What exactly is density? Density refers to the number of hair strands per square inch of scalp. It is classified as low (thin), medium, or high (thick). So when we say, "Oh she has very thick hair," we mean there's a lot of strands of hair on the person's head.
Here's something to consider--just because a person's hair is thick, doesn't mean it's coarse! "Coarse" (or fine and medium) describes the diameter of a single strand of hair--this is known as hair texture. Density is the measure of how many of those single strands are on one's head. So, a person can have very fine hair (each strand is small in diameter), but also thick. Likewise, one can have coarse hair that is thin in density (there are not a lot of strands of hair).
It's important to know your density for proper styling techniques. Low density hair tends to be weighed down by heavy products while thick hair may do just fine. Weaves and braids are excellent protective styling options, but only when installed correctly. You never want to add to much braiding hair or too many tracts to hair that is fine and thin--the follicles will not be able to support the extra weight which WILL result in breakage!
Hopefully it's a bit clearer why stylists must to take into account your density for product and styling recommendations. How would you describe your hair's density?