Ceteareth-20 helps other ingredients to dissolve in a solvent in which they would not normally dissolve. Ceteareth-20 functions as an emollient and emulsifier. It’s usually used in conjunction with other alcohols and fatty acids, working to thicken a solution and help other ingredients dissolve in a solvent. It also functions as a non-ionic stabilizer in oil in water emulsions. This ingredient is one of the most common emollients used in cosmetics.

Ceteareth-20 (Wikipedia)

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The INCI names ceteareth-n (where n is a number) refer to polyoxyethylene ethers of a mixture of high molecular mass saturated fatty alcohols, mainly cetyl alcohol (m = 15) and stearyl alcohol (m = 17). The number n indicates the average number of ethylene oxide residues in the polyoxyethylene chain.

Chemical structure of ceteareth-n, where n is variable from 2 to 100 and m is 15 or 17

These compounds are non-ionic surfactants that work by attracting both water and oil at the same time, frequently used as emulsifiers in soaps and cosmetics.