Photons are the elementary particles that make up light. They come in different wavelengths, from long-wavelength radio waves that bring you your favorite music, to short wavelength, high-energy X-rays that doctors use to see through your body. ToF sensors use wavelengths somewhere in the middle of the electromagnetic spectrum, close to visible light.
This is a companion discussion topic for the original entry at https://community.robotshop.com/index.php/blog/show/a-brief-introduction-to-time-of-flight-sensing-part-1-the-basics-1