When Magento is performing some tasks, it temporarily creates this maintenance file. Magento checks for its existence and if it’s there will send users to the page you described.
It’s supposed to automatically delete this file when done processing whatever task it was doing, but I’ve experienced occasions where something went wrong and it failed to delete it.
If you try to delete orders in the backend, you will find out that you can only set the status to “cancelled” and the order is still there. Unfortunately, Magento doesn’t allow us to delete those via administration, so you will not find any “Delete order” button. This can be quite frustrating both to developers and the merchants.