For designing a system, you nearly always use the worst case, or December-January map. "Flat plate collector" is simply a solar panel.
The full set of maps ( all 300 or so) are available at the NREL website:
Translation: At high noon on a clear day, each square meter receives 1000 watts of sun power. If you look at the large yellow areas, you will see that it gets around 6,000 watts on an average day. So, even though the average day is exactly 12 hours, the power you actually get on your panels is equal to about 5 to 6 hours of full sun per day. Since the typical modern solar panel is about 12% efficient, you will get about 700 watts per square meter of panel. So, if the map says that you live in a "six" area, you can expect sun power equal to 6 hours per day over the entire year.
A large portion of the country is now yellow, showing that good solar power is available for most of the country during the summer.
As you can see, in the winter it's an entirely different story. Much of the country now gets an average of 4 hours or less of full sun hours per day.
Compare this to the previous map, and you can see what a difference a tracking mount system can make. The example shown is for 2-axis tracker, like the WattSun. A single axis tracker, like the Zomeworks, will be a little less, but still considerably more than a fixed array. The biggest problem with tracking mounts is that they give the biggest increase in the summer, while the biggest need for power is in the winter. If you have plenty of power in the summer, but fall short in winter, your best option may be to use a maximum power point tracker. Tracking mounts used to make a lot more sense when solar panels were selling in the $10+ a watt range, but with current prices around the $4.50 to $5.50 per watt range the economic advantages of tracking are less.