dB is an abbreviation for decibel.
http://en.wikipedia.org/wiki/Decibel
It is a useful mathematical form, used to express ratios (one value divided by another). Pay attention to the presence or lack of a suffix... When you see just "dB", a simple ratio is being expressed, the two values should be identified in the context of the value.
When you see "dBm" the value expressed is relative to a standard power level of 1 milliwatt (1/1000 Watts).
There are many other suffix possible. A quick search will usually yield a definition.
Here is a very basic 'how-to' that may help you translate the information in your TVFR into an educated estimate of how much gain you need an antenna to have in your application.
http://forum.tvfool.com/showthread.php?t=109