There is a switch above my thermostat in the hall. I was told this is for a principle house fan. Can you explain when I should use this fan?

The principle air fan or whole house fan is an inexpensive air exchanger. Most of the time this fan is located near the furnace but pulls air out of the home from a central location in the upper level of the home. Here is the premise of how it works. When you have high humidity in your home or undesirable odors such as smoke from cooking, you can turn this fan on to exchange the stale inside air for fresh outside air. When you turn on the fan it is linked to the blower on the furnace. By using this fan you are removing air from the home so the furnace blower is used as a mechanism to replenish this air. The fresh air is cold in winter so most people only use this fan when necessary. A central air exchanger is a better option but much more expensive.