A market order is a buy or sell order which will use the current price of the currency (based on recent orders) to adjust the sell/buy price. This option allows you to skip having to propose a price but you may not get the best deal for your exchange, but it is highly more likely that a trade will be executed quickly.
When you place a market order, the instruction is to fill your order at any rate in which the market (other traders, not the exchange) will provide. Market orders should only be used when one wants to liquidate a position at any cost.
When you want to execute a market order, select the trading pair and make sure to select "Market Order".
Also read about Slippage