Short for plebeian, a disrespectful term originally used to refer to lower class people or commoners in ancient rome. Could nowadays be used similarly disrespectful to describe someone who doesn't "get" good art, food, music or anything culturally relevant to you, as well as uneducated rude people in general. Can also be used half-jokingly in friendly banter.
"Wow you're such a pleb."
"Don't worry, your track is a banger. They're just plebs"
"Damn, this place is way too fancy, I feel like a pleb."