A market order is an order type that executes immediately against the best price available. As long as there are willing sellers and buyers, market orders are filled. A market sell will match the best available bids on the order book, and a market buy will match against the best available asks on the order book.
Market orders are often used when rapid execution is prioritized over the price at which the order executes. When placing this order, the trader prefers the trade to happen immediately at the current best price available.