Esh is an joke expression popularized by Faya Braz and Swish Beatbox. In native French, the expression "wesh, ma gueule?" is the English equivalent of "yo, what up?" Wesh meaning yo, the phrase was simplified to just "esh."
Esh is also a common beatbox sound, a vocal snare when said quickly. As a joke, Faya Braz used it as a way for beatboxers to say hello to each other. This quickly caught on and the community quickly embraced it and are incorporating to everyday words, such as "fr-esh."