INCI: Squalane
An emollient — it fills the gaps between skin cells so the surface feels smoother. Light, fast-absorbing, doesn't leave much residue.
Note the spelling. Squalene (with an "e") is unstable and oxidises. Squalane (with an "a") is the hydrogenated, shelf-stable version, and that's what's in cosmetics. Most is now made from sugarcane or olives rather than the shark liver it was historically taken from — if that matters to you, look for "plant-derived" or ask the brand.
On a label: mid-list, often alongside other emollients.