It means 'loser'
It comes from the word 'jobber' who is a type of pro wrestler who's role is to lose and make the other guy look strong going in to more meaningful matches. (Synonyms: jabroni, enhancement talent, job guy, carpenter, fall guy)
Often these are young wrestlers cutting their teeth in the business. But there are some wrestlers who specialize in the role.
Note: some wrestlers (jobbers) find the word 'jobber' to be insulating, insinuating they are in the role due to lack of talent. The term enhancement talent is considered more respectful.