I guess a micro oven is a very small oven. For cooking very small things.
If you mean microwave oven, see https://en.wikipedia.org/wiki/Microwave_oven. In short, a magnetron generates a few hundred watts of radio waves at 2.4GHz. This heats up the food. It all takes place within a sealed box, that acts as a Faraday cage, so the radio waves don't leak out.
They have been around for decades, and are quite safe so long as you don't deliberately defeat the safety switch on the door. I have been using them for 30 years, and they haven't done me any harm yet.
