What is an Antenna Tuner. Exactly as it implies. It tunes the antenna. Without getting too technical, Radio waves will jump off of your antenna most efficiently when your antenna is at the proper length for the frequency your are operating on. When the antenna is too short or too long the efficiency is lost and reflected back in to your radio. This is called reflected power. The radio's antenna feed line impendence is 50 ohms. This is why we use 50 ohm coax. With the proper length of antenna we get a proper 50 ohm load. When the antenna is of a different length than it should be, that impendence changes. Once again this results in that percentage of reflected power. Through the use of inductors, an antenna tuner will restore the line impendence to 50 ohms removing the threat of the reflected power, due to the use of an antenna not tuned for that particular frequency. In simple terms, it allows you to "Tune your SWR flat" on any antenna.
How can this be used in the real world? A tuner will allow you to use just about anything as an antenna. Or, use just about any antenna on other bands. Example: I used an MFJ Antenna Tuner to use a cb radio base antenna to make a contact in Serbia on Ham Radio. Meaning, if you are a Ham that currently only owns a cb radio antenna, with a tuner you can use the cb antenna to talk on ham radio. A tuner will open you to hundreds of different experiments. I once hooked coax to the gutter on my house and used the gutter as an antenna. An antenna tuner is a must on ham and short wave radio. You will often hear of people using "Long Wire" or "Dipole" Antennas. They place wires as long as possible on their property and then use the Antenna Tuner to match it up to the transceiver. Before transmitting to tune the antenna, the user turns the inductor knob and the tune knobs on the tuner first to bring up their receive to the highest level by ear, then the antenna is tuned with the transmitter. When you get good at it, you can tune the antenna by ear alone, without the need for transmit tuning.
Other Great features of Antenna Tuners. MFJ tuners often have multiple inputs. My MFJ 949E has places to plug in 3 antennas. Two coax fed antennas, long wire or 450ohm ladder line. The knob on the front acts as an Antenna Switch. With the flick of a knob I can choose between 3 antennas. My MFJ Tuner also has a dummy load. The tuner also has bypass settings where antennas can be plugged in that need no tuning at that time or at all. It really is a wonderful tool. Most have a "peak reading" watt meter.
And finally, a tuner will help you on cb if you can't get to your antenna to tune it properly. An SWR of 2.1 can cause your radio to get real hot. A tuner would allow you to tune it until such a time the antenna can be tuned itself. A tuner has so many uses it would take pages to explain them all.
MOST MFJ TUNERS HAVE PEAK READING WATT METER BUILT IN