The Full Form of SLES is‍ Sodium laureth sulfate.

Sodium laureth sulfate, an accepted contraction of sodium lauryl ether sulfate, is an anionic detergent and surfactant found in many personal care products. SLES is an inexpensive and very effective foaming agent. 

Depending on manufacturing processes, sodium laureth sulfate may be contaminated with measurable amounts of ethylene oxide and 1,4-dioxane. ⁱ The International Agency for Research on Cancer ethylene oxide as a known human carcinogen and 1,4-dioxane as a possible human carcinogen. Ethylene oxide can also harm the nervous system ⁱⁱand the California Environmental Protection Agency has classified it as a possible developmental toxicant based on evidence that it may interfere with human development. ⁱⁱⁱ 1,4-dioxane is also persistent. In other words, it doesn’t easily degrade and can remain in the environment long after it is rinsed down the shower drain. 1,4-dioxane can be removed from cosmetics during the manufacturing process by vacuum stripping, but there is no easy way for consumers to know whether products containing sodium laureth sulfate have undergone this process. ^iv

The industry panel that reviews the safety of cosmetics ingredients notes that sodium laureth sulfate can irritate the skin and eyes (though approving of its use in cosmetics). ^v