Good till canceled
GTC, as it's usually referred to as, is pretty easy to understand. When an investor or trader uses a limit order or another type of trade where there is not immediate execution, adding the good til canceled condition keeps the order active after the trading day ends.
Use to Investors
GTC is a useful tool that can allow investors and traders peace of mind. For instance say you are going on vacation to the Cayman Islands and would not like to have to worry about one of your more dubious holdings while you were there. You could place a stop-loss order on that stock with the GTC condition and not worry about it plunging to zero there is no guarantee you will get your stop price although in many situations it will usually be close, barring a unusual volatility and after market surprises) while you weren't able to watch it.