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 want to make a market order, select the trading pair you want and make sure to select "Market Order" out of the three options at the top of the buy/sell window.