Microwaves are electromagnetic waves of the same class as radio and light.
Like radio and light waves, microwaves can be sent through space and received or absorbed at a distance. The space may be a vacuum or may be filled with certain gases, liquids, or solids. Most materials have some degree of absorption so that a significant portion of microwave energy is consumed as it passes through. The material is then heated by this absorbed energy. The frequencies at which microwaves are generated are quite high in terms of normal human comprehension.