It is a German term meaning "low-earner". It basically describes a person that has a usually simple job that does not pay well or is basically poor. Since around
2019 the term has been developed into an insult towards people who don't have
money. You
may use it towards a person because of a specific situation, in which a person shows behavior of having a low income, or you use it to describe one's general life situation. It is usually meant in a funny way but
may also deeply insult a person.
*two friends want to buy beer: Friend 1: "I don't have enough
money on me right now so I
will only buy the cheap brand like Karlskrone." Friend
2: "Hahah Geringverdiener!"
*guy with a Porsche is annoyed by a cyclist in front of him: Guy: (honking and overtaking him): "out of the way Geringverdiener"