F/B ratio is one of the parameters that describes how the antenna behaves. Generally, the higher the gain of any given antenna, the higher the F/B ratio. There's more to it, but that's a very simplified explanation.
In a TV antenna the front to back ratio does not matter unless there is interference or reflections that arrive at the antenna from the back side. Antenna gain is more defined by the width of the pattern off the front of the antenna.
