Market orders are the most straight forward way to buy and sell shares. They are aptly named because, just like in a market, you're buying and selling shares off other people in real time (or when the markets next open).
Market orders are completed as soon as possible at the best available share price
What does that mean? If you're in Hatch and you look at a company (say, Lululemon) you'll see the current share price, which at time of writing is $235.90 USD per share.
The share price changes every time an order is placed, so the price you see when you place the order (e.g. $235.90) is just an estimate. When you place a market order to buy or sell shares, your order will be matched to best available opposite order to give you the best price. This is all done in real time, and the shares are traded a lot, so the share price usually doesn't change that much from the estimate you see in Hatch (especially if the US markets are open).
Placing market orders when the markets are closed
When you place a market order while the markets are closed, it will be completed at the opening share price. Since off-hours trading and demand for shares can change overnight, the price may change a bit by the time the markets open. Sometimes it might only be a few cents, but it can change more if a major overnight event has a big impact on demand for the shares.
For example, the Facebook/Cambridge Analytica scandal made news on Saturday March 17, 2018. On Friday the 16th, the day closed with Facebook sitting at $185.09 a share. When the markets reopened on Monday, they had a significant drop down to $177.01 a share, because demand for the shares dropped significantly overnight due to the bad news.
Obviously those events are unusual, and buying shares for less than you expect can be a good thing (Facebook's share price is sitting at $212 at the beginning of 2020). If you want to make sure you only buy shares for a price you specify, you might be interested in limit orders.