SLS is a chemical surfactant & stands for sodium lauryl sulphate and can also be called:
sodium dodecyl sulphate
sodium monolauryl sulphate
Its used in cleansing/cleaning products because it can break down fats and grease so they are easily washed away.
SLES is made from SLS but it’s a bit gentler.
SLS & SLES are bad for skin because they breakdown skin sebum and strip the skin of its natural grease, which fills the gaps between skin cells. This allows water to escape causing dry skin, eczema, or even oily skin.
If your skin cannot produce the natural grease fast enough to plug the gaps or is continually being washed away, water will escape at a much faster rate than your body can replace in the affected section of skin.
And sometimes the skin tries to replace its lost natural sebum by producing too much oil, and therefore leaving you with an oily complexion.
Products such as liquid soaps, shower gels and shampoos usually contain the highest concentrations of these ingredients.