A limit order is an instruction to buy or sell only at a price specified by you. You would use a limit order should you want to execute a trade at a certain price. You can input the specified price and should the instrument reach that price your order will automatically be executed.
Should you want to cancel the limit order before it has executed you can do so. Simply go to your pending order on your dashboard and select cancel.
Please note: the limit price must be a minimum of 1% away from the current price of the instrument.