A pending order is an instruction to open a position when an instrument reaches a certain price pre-defined by you. You can place a pending order via the main 'Order' dialog by selecting the 'Limit' or 'Stop' tabs. The dialog will then allow you to set the limit price and expiry date/time for the pending order.
Bear in mind that Buy stops & Sell limits must be placed above the current market price while Sell stops and Buy limits must be placed below the current market price.
Alternatively, you can instantly place a pending order by right clicking on the chart.