INCI: Glycerin
A humectant — it pulls water toward itself. In a formula that means it draws moisture into the upper layers of the skin and holds it there.
It's in almost everything, it's very cheap, and it's one of the most studied cosmetic ingredients there is. Its ubiquity gets mistaken for it being a filler. It isn't; it's usually doing real work.
On a label: commonly second or third, right after water.