Giving a task or project a start date means you don't want to work on this until the specified date.
Anything with a start date in the future will show in the backburner and is hidden from the Master List.
Adding a start date
To add a start date, click the edit icon on the right and pick a start date in the mini dialog.
In the backburner any item that is there because it has a start date in the future will show the start date underneath the title:
When the start date arrives, the item will automatically be moved out of the backburner to the Master List.
Any item that got moved to the ML will have a "new" tag.
Clicking the "new" tag will make it go away, meaning you acknowledged that this item became actionable and performed the next step in your workflow (e.g. scheduling it in your days, planning it for a specific day etc.)
Check out all of Marvin's date options for an overview on dates.