There are so many different Amateur Radio bands on the HF Amateur Radio frequencies and that it makes it difficult to put up a separate radio antenna for each frequency we might want to use. Many hams use multiband antennas but these antennas will usually narrow the frequency spectrum the antenna will work at before the Standing Wave Ratio (SWR) becomes so high it can cause distortion to the transmission or damage the transmitter.
To overcome the problem ham radio operators frequently use an antenna tuner between the transmitter output and the antenna. Antenna tuner is a bit of a misnomer because they are actually do is match the impedance to lower the SWR at the transmitter rather then antenna tuning. I found a video that gives an explanation of what an HF antenna tuner is and how to tune the tuner.
To watch the video [CLICK HERE]