Market Orders allow you to buy or sell the security immediately during market hours (Stock Market hours are Monday-Friday 9:30 am - 4:00 pm ET). Market orders are entered in real-time and are therefore not accepted outside of market hours (or when trading a certain stock is halted or suspended, subject to market conditions).
If you do place a Market Order outside of market hours, SoFi automatically adjusts your Market Order to be a Market Order on Open. You will see your trade pending until the following market day. Keep in mind: due to after-hours trading, the price you see today may change overnight! You will get whatever the stock is trading at when your order goes through as a Market Order on Open.
Note: If you place an order on Friday, the next market day would be Monday (with the exception of holidays).
SoFi also offers limit orders, fractional orders, and Market Order on Open, which is typically able to be traded even outside of normal market hours.