They are called pre-amps or preamplifiers. The are placed at the antenna to boost a signal of a particular frequency or range of frequencies. Sometimes they are used to make up for lost signal on long coax runs. In most cases they are not needed, and it would be best to spend the money on a good antenna system. If you are trying to monitor a distant system in the near microwave frequencies, such as living 50+ miles outside of a large city, and you are haveing problems, use a directional antenna and then move to a preamp if needed. Dont overlook the use of good coax as well, especially when dealing with the near microwave frequencies of some of these new digital systems being used i.e. 700mhz +.