What does 'get a job' mean?

It means "find paid employment".

any other meaning? I mean..like a metaphor?

A term used in place of get a life. "Get a job" is a phrase similar to leave me alone or go away. Guy: Hey, how are ya? Girl: Get a job.
From "http://www.urbandictionary.com"

So it's a slang that even native speakers don't know.

CharmYouSo it's a slang that even native speakers don't know.

Perhaps. I don't get out on the city streets much these days, but I wouldn't trust the Urban Dictionary, either. Suffice it to say that it is not established slang like "Get a life." Besides, you did not give any context. It might have been in something like, "My mother said I should get