Market order allows you to immediately buy or sell a given asset at the best available price.
Take a look at the screenshot of order book. The best available price is displayed in the top line, highlighted in red: 9343.48 USDT per BTC. If you set up a buy market order at this moment, you'll be paying this price for your trade.
But there's only 0.04516 BTC available at that price. If you buy slightly more than that, you'll be paying the second best price of 9343.49 for that "extra" amount.
And if you were to buy, say, 0.18 BTC, you would in fact purchase:
0.04516 BTC for a price of 9343.48 (first line in the order book)
0.11 BTC for a price of 9343.49 (second line in the order book)
0.02484 BTC for a price of 9343.66 (third line in the order book)
0.18 BTC total
You now see: the actual price you pay depends on the market situation and the amount of supply.
The price changes in real time. Because of this, the total order amount and trading fee are shown as estimates rather than exact values.
Stop order: helps you prevent the losses
By checking the "Stop" checkbox in the order creation window order, you can create a stop-order. You'll be asked to specify a "stop price".
Stop order will get activated and executed only when the current price equals the stop price or worse.
Stop order doesn't require reserved funds. It does not appear in the Active orders list and the order book until it's activated.