A showcase gig, or just a showcase, is an introduction to an audience for a new act. Sometimes, labels use showcase gigs to get their new signees in front of the press, while other times, unsigned acts play showcase shows in the hopes of impressing someone in the industry enough to get a deal.
Showcases can be handy for labels or others who have enough pull to get the right people out to see musicians, but beware showcases that charge unsigned musicians a fee to pay. Some of these showcases charge thousands of dollars for minutes on stage, and there is absolutely no guarantee that anyone who can do your music career any good will be in the audience. In fact, chances are, they won't be. If you are tempted by a paid showcase opportunity, do your homework and find out who attended past events and whether anyone has ever had success finding a deal at that particular event. Most worthwhile showcases, like showcases at music industry trade shows, do not charge musicians to play.