Density refers to the amount of hair on a wig or hairpiece. It is usually measured by the number of hair strands per square inch of the cap. 120% density means that there are 120 hair strands per square inch of the cap, making the wig have a natural and medium volume. Wigs with 120% density are popular because they provide enough hair volume without being too thick or heavy. They allow for a natural-looking hairstyle while still providing coverage and fullness. However, density preference varies depending on individual's preferences and desired look.