What does market order mean?
Market orders are the most immediate way to buy or sell a financial instrument to enter or exit a trading position: you buy or sell at the best currently available price. This differs to a limit order, which allows you to specify a price.
What is the bid/ask spread?
This difference between the bid and ask/offer price is called the bid/offer spread or the bid/ask spread. What are the bid and offer prices? Remember the rule, 'buy low, sell high'.
Buyers want to pay the lowest price they can. So buyers bid low.
Sellers want to get the highest price they can. So they offer to sell at a high price.
Market order prices are based on the bid/ask spread
When you place a market order to buy or sell, you agree to pay the price the other side wants, in exchange for getting filled as soon as possible.
Buyers pay the price the seller is offering - the offer (or ask) price, which is the higher of the two. Sellers will sell at the bid.
Because of the bid/ask spread, you may find yourself slightly in the red when your order is filled. This reflects the initial gap between the price you entered at and the price you would pay to exit at that moment.
Nadex market orders have protection against slippage so you only get filled at or near the price you expect.