Outside of knowing your market order will execute immediately (as long as the market is open), you can also create a limit order that will execute at or near a price you specify. Here’s a little more information about trade types:

A market order is a request to buy or sell a stock at the current market price. If you enter an order during market hours (9:30am – 4:00pm ET), we process it immediately.

A limit order is a request to buy or sell a security once a customer-specified price is reached or surpassed. When this happens, the trade will execute.

A stop-loss order is a request to sell a security once the market price reaches or falls below a customer-specified price. When this happens, the trade will execute.