This is the door you would use to define the bagua, or the feng shui energy map of a home.
As with everything, there are many variables to this statement. Generally, unless the house went through a big remodeling project and the main door was repositioned by the architect taking into consideration the house as a whole, the front door is as mentioned above.
In many cases a back door, a side door or a garage door would be used much more frequently, which makes it important to have these doors infused with good, clean energy for you and your family's well-being.
This does not make any of these doors a front door, though; just a more active door closely connected to your health.
What does it mean to have your secondary active doors infused with good energy? Basically, it means having a welcoming secondary entry that you treat well. An entry that looks good, is clean, well-organized and visually pleasing to the eye/well decorated.
As you use that door everyday, on a feng shui energy level the way you come into the house and the way you leave the house are very important.
In my practice I often see big beautiful homes with impressing main entries that are rarely used. This is not the biggest concern, though, as long as the front door is used at least from time to time (e.g. for welcoming guests, or post deliveries, etc.)
The concern is when people disrespect their own energy and well-being by entering their home through ugly and over-cluttered garage doors, or side doors that open into busy laundry rooms. Entering your house through a low energy door is bad feng shui for you.
It is important to understand that energy comes into a home through any openings: doors, windows, skylights, etc. The main Chi flow, though, or the strongest Chi flow, comes through the front door.
This is why the front door is called "the mouth of Chi", as the house absorbs its main nourishment through the main, or the front door.