Serums have risen in popularity over the past three to four years—and now, they’re everywhere. Name any skin concern, and you can bet there’s a serum out there that can help you address it.

But if you don’t know much about serums (which is most of us, unless you’ve seen a dermatologist, skincare expert, or read a ton about skincare), you may not understand the hype. Why are they so important? What even are they?