This term actually refers to a London stockbroker who only buys and sells shares from other stockbrokers. In South Africa, the term is used to refer to any broker who buys and sells shares for himself, and even more broadly, anyone who buys and sells shares quickly, looking for short-term profits.
Another definition is - A jobber is a distributor who buys wholesale and sells retail. Typically their margins can vary from 15% to 45% depending on the industry.