A synthetic big copolymer molecule that is used mainly as a solubilizer (make small amounts of oil-loving things soluble in water-based formulas) and gelling agent.
In general, poloxamers are interesting big molecules composed of three blocks: the middle block is an oil-loving part (from propylene oxide units, if you're into chemistry), while the left and right blocks are two water-loving parts (from ethylene oxide units). This means that poloxamers are partly water and partly oil soluble and thus they are surface active agents acting as emulsifiers and/or cleansers. The size of both the oil and water-soluble part can vary, and the numbers in the name of the molecule refer to both the overall size of the whole molecule and to the ratio of the water-soluble part.
This particular guy, Poloxamer 407, is a big one with significant water-soluble part (70%) hence it works as a solubilizer to make small amounts of oil-loving ingredients (such as essential oils) soluble in water-based formulas. It is also the best gelling and body giving agent out of the poloxamers commonly used in cosmetic products.