A GTC order persists day after day. Brokers usually cap it at 60 to 90 days. It is the opposite of a day-order, which expires unfilled at the end of the session.
GTC is convenient for resting limit-orders at prices you would be happy to trade, but stale GTC orders are a classic source of surprise fills weeks later.
Example: you place a GTC limit buy at $18 on a stock trading at $22. Three weeks later it gaps down through $18 on bad news and you are filled, whether or not you still want it.
Related: day-order, limit-order, fill