A market order is the most straightforward type of order. It executes immediately against the best price available. As long as there are willing buyers and sellers, 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 you need to prioritize speed over price, for example, when taking advantage of a fast-moving bull run. You'll use this order when you need your trade to execute immediately at the current best price available.