RF waves are a form of radiation. Radiation is the release of energy in the form of electromagnetic waves.
It can be classified as low energy or high energy depending on the amount of energy released. X-rays and gamma rays are examples of high energy radiation while RF waves are considered low energy.
Radio waves, WiFi, and microwaves are all forms of RF waves.
The form of radiation used in RF skin tightening releases about 1 billion times less energy than X-rays.