"Meh" is a casual response that means "this does not matter much". I don't know that it's proper English by any means, but it is certainly a common colloquialism. A cursory search suggests that it's imported from yiddish.
It's not an uncommon expression. I searched the term in a chat room my friends (of varying professional backgrounds) have used over the past 5 years. There were about 200 instances of us using the word.
Native English speaker here. It’s an extremely common expression in some regions. The main character of the Emoji movie is named “meh”.