The Brief: Salty refers to someone who is bitter about or upset over something.

DEEP DIVE
Salty refers to someones who is bitter about or upset, usually over something small or inconsequential. Being salty, or “saltiness” is often a result of being made fun of or having shade thrown at you.
I’m so happy our parents didn’t have Facebook to post every time we went to the dentist, were potty training, tried a new food, received a report card, etc. #toomuch #salty #notappropriate #whatevenissocialmedia
— Danielle Marie (@happily_d_) February 1, 2019
Don’t be salty mate, I know bitter is your default setting but there is no need.
— WalkerlyYT (@walkerlyYT) February 1, 2019
The slang term actually dates back to the early twentieth century, in relation to sailors, though it has become more popular on social media and text lingo in recent years. It is sometimes used with a hashtag: #salty.