A position is an open transaction. You've bought something, and you're holding on to it, but you haven't sold it yet. A buy transaction is a long position, and a sell transaction is a short position. The size of a position refers to how much currency has been bought or sold. When you initially buy or sell, you "open" the position. When you are done with the trasaction, you "close" the position.
When you have an open position, your MetaTrader software will show you how much you will gain or lose if you close it. It is advisable to always use a stop loss on every order. If you need to add a stop loss to an existing order in MetaTrader, right click on it and choose "Modify or Delete Order." For open orders, you can't change the price you entered at, but you can plan for the price you will exit at by setting a Stop Loss and/or Take Proft.