The market order involves immediate buying or selling of the asset at the best available price. Market orders, unlike limit orders, prioritise the certainty of execution over the quality.
The more buyers and sellers are quoting prices in the order book (e.i. the more liquidity there is), the easier it is to execute the market order at more favourable prices.
To create more liquid order book, exchanges, typically, charge higher fees for market orders than for limit orders.
A trader performing a trade with the market order is considered to be a taker.