A clear, colorless oil-like liquid that makes the skin feel smooth and nice (aka emollient) and it does so without it being greasy.
What's more, it can even reduce the heavy, greasy feel in products with high oil content. It's also fast-spreading meaning that it gives the formula a good, nice slip. It absorbs quickly into the skin and helps other ingredients to penetrate quicker and deeper.
Thanks to all this, it's one of the most commonly used emollients out there. There is just one little drawback: it has a high comedogenic index (5 out of 5...), so it might clog pores if you're prone to it.