Multipath signals are signals reaching the antenna indirectly, from bouncing off other objects like hills, buildings, trees, airplanes, vehicles or anything metallic. The signals arrive at the antenna at slightly different times, which can confuse the tuner and make it hard to keep a lock on the signal. In the analog days, multipath displayed on the screen as "ghosts" or outlines of the original image shifted slightly to the side. With digital, multipath can cause lower signal and when bad enough, may cause A/V dropouts. Some tuners' multipath-rejection capability are much better than others.
Re your TVFool, I found the original one you posted in the other thread. This one appears to be identical. Was it based purely on address entry or were the results pinpointed over your house, using the mapping tool?