You may have noticed that there is a delay for orders placed the previous night to appear in your portfolio. We call these orders that go into a queue to be filled the following morning "market open or MO orders".
Remember that there is a 20 minute delayed datafeed, so even though the market is open at 9:30am EST, we still don't have an active quote for any particular stock until 20 minutes later (at minimum). As well, not all stocks start trading with significant volume right at the opening bell. As a result, you'll see market open orders filled between 30-60 minutes after the market open.
Don't worry if the price of the stock moves during the time when your order is still open. The price we fill at for all market open orders is the open price. This isn't completely realistic to what you'd get in real life, but it's the best way for us to make sure that all users get a fair price.