Gig: Slang for storage of a billion bits of information, more or less. That's our economy today, with digital companies like Facbook, Google, and others comprising the highest valued companies on the planet. But that said...when you're a freelancer, you work based on contracts or gigs, like a musician would.
This is known as gig economy. It's work that is temporary, such as freelance, contractors, and temp services. Uber and Airbnb are two companies that are considered a part of the gig economy, because people work or earn wages based on a gig.
Companies use these types of employees as opposed to hiring full-time employees as a cheaper (and sometimes more efficient) option. This economy tends to make it harder for full-time employees to fully develop in their careers, as more and more companies are turning to temporary employees.