An antenna tuner increases the frequency range (the band of different wavelengths) and reception of your CB radio, meaning that you can receive more signals over greater distances. You can build your own homemade antenna tuner.
When a radio antenna is the correct length for a particular frequency, its reception is better. The function of an antenna tuner is to match the impedance -- defined as the resistance to the flow of electricity -- of an antenna to the impedance of a radio. This creates an effect similar to having an antenna of exactly the right length for any given frequency.
An antenna tuner allows your radio to pick up a wider range of frequencies, meaning that it can tune in to more signals. It also increases reception so that transmission becomes much clearer and signals that were previously too weak can be picked up.
The exact components vary between designs. To make a homemade antenna tuner, you will need an inductor, which you can make from insulated wire coiled around a nonconducting core such as a section of plastic pipe. A metal core will not work. You will also need a variable capacitor and a rotary switch to adjust the tuner. You can house the tuner in a case or leave it open-frame to make adjustments easier.