An affirmative response often used by neurodivergent people, derived from 'ye', a shortening of 'yes'. An e used in this manner is always lowercase.
Person 1: Yo, you comin to game night tonight?
Person 2: e
Person 1: Great! See ya there!
When in discord the letter E can refer to asking if somebody wants to voice chat.
"Hey man, wanna E?"
"Sure, let me get my mic set up!"
or
"E?"
"Sure!"