Investors who want to buy or sell a stock at a certain price can enter orders that remain in effect until executed or cancelled.
For example if you own the stock of the XYZ company and would like to sell it for $50, but the stock is trading at $45 you can enter an order to sell at $50 and the order will stay on the books until it reaches $50 or the order is cancelled.
This kind of order can also be used to sell a stock if it drops to a price below the current market. It’s usually referred to as a “stop-loss” order and is used by investors who want to limit their losses in case their stock declines.
There are advantages and disadvantages to GTC orders so be sure to ask your advisor what they are.