Porosity refers to how susceptible your hair is to water: basically, how well the outer layer of the strand absorbs or doesn’t let water through.

“The outer layer of the hair strand is called the cuticle layer. The cuticle layer is made up of little tiny cuticles that overlap a little,” explains hairstylist Danielle Melary. Think of it like shingles on a roof, and your porosity depends on how tightly those shingles are packed together.

“Hair porosity describes how the hair cuticle absorbs and holds moisture in its pores—hence the term porosity“says hairstylist Miko Branch, co-founder of hair care brand Miss Jessie’s. So when your hair is high porosity, it means it’s very porous, so the cuticles are more loose than tight, so they’re able to absorb a lot of water.

Conversely, you can have low-porosity hair: “If someone has low-porosity hair, it tends to have trouble absorbing moisture because the hair is water-resistant,” Malary says. “When hair has low porosity, the cuticles are pressed so tightly together that it is very difficult for moisture or water to penetrate the strand.”