This is the asset pairing you are intended to trade. It reflects the price of one currency, displayed in the second currency value. The drop-down list in this field displays all tradable options at the nominated Exchange.
For example, The Trade Pair BTCUSD displays the price of Bitcoin in US Dollars. Whereas XRPBTC details, the price of XRP (Ripple) BTC (Bitcoin) value.
Trade Pairs also signify an intent to purchase Bitcoin in US Dollars (in the case of BTCUSD).