Despite it’s name, an antenna tuner unit (ATU) doesn’t actually tune the antenna, or indeed any part of the antenna system. Now you well may be asking yourself, “If it doesn’t tune the antenna, why use one?”
The reason you use an ATU is to “fool” the radio into thinking that it’s delivering the RF output into a nicely matched load. When modern solid state radios are trying to send RF into a mismatched load, the built-in protection circuits kick in and lower the output accordingly.
When transmitting, RF power sent to an antenna will always have a small proportion reflected back to the transmitter.