This is done by matching the impedance of the radio to the antenna. An antenna tuner matches a transceiver with a fixed impedance (typically 50 ohms for modern transceivers) to a load (feed line and antenna) impedance which is unknown, complex or otherwise does not match.
An ATU allows the use of one antenna on a broad range of frequencies. An antenna and atu are not as efficient as a resonant antenna due to feedline losses due to the SWR (multiple reflections) and losses in the ATU itself. An ATU is an antenna matching unit, and cannot change the resonant frequency of the aerial. Similar matching networks are used in other equipment (such as linear amplifiers) to transform impedance.