A changing magnetic field generates an electric field, and a changing electric field generates a magnetic field. This is precisely what electromagnetic radiation is. A source charge simultaneously emits changing electric and magnetic fields which then continually feedback each other. This is how radiation can propagate through empty space.
The electromagnetic spectrum consists of all the possible wavelengths for electromagnetic radiation. Waves with long wavelengths have low frequencies and hence carry low energy. For example, radio waves typically have a wavelength between 1 mm and 100 km! The visible spectrum of light is only is a from about 400 to 700 nanometers. Electromagnetic waves with short wavelengths have high frequencies and are dangerous because of their incredibly high energy, for example, X-rays and gamma rays.
(Wavelength image from Universe by Freedman and Kaufmann.)