Luc, each frequency in the spectrum has a "wave length." Wave length can be calculated by dividing 300 by the frequency in megahertz.
For example, 15.000 megahertz is 20 meters (wave length) (300/15 = 20 meters).
Radio signals are a sine wave. The length of one cycle of the sine wave is the "wave length."
Antennas typically are most efficient if the length is 1/4 wave length. So, ideally, the antenna length for 20 meters would be 20/4 = 5 meters in length.
This is difficult to achieve in an aircraft where the antenna must be fixed length, but different frequencies are used.
So, to compensate for the fixed length and varying frequencies, an antenna tuner "electronically" adds or subtracts to/from the length of the fixed antenna.
This is a very simplified explanation, but if you have had some trigonometry hopefully this will make a little sense.
Check 6