Explanation of a market order
Basically, a market order is a very fast sell or buy request at the most favourable current price available.
To be fulfilled, a market order requires liquidity. The base for its execution are the limit orders that have been already put on the order book.
But in opposition to the limit orders that are placed on the order book, the market orders are processed immediately at the present price. This means you are a market taker and you will pay a fee accordingly.
Using market orders on Binance
Once you log in to your Binance account, select the market of your choice. Let’s say you decide on AION/BTC.
Choose the “Market” tab in the trading interface section.
Next step is to write the amount of AION you are interested in.
The last thing is to click “Buy AION” button. The procedure will look very much similar if you decide to sell AION. The only difference is that in the end you must click “Sell AION” button (of course you need to have some AION in order to sell it).

At this stage, you should see a note with the confirmation of the transaction on the computer screen. The market order you have selected has been triggered.

to know more visit : When and how to use market orders on Binance - Binance Wiki