Fresnel diffraction is a fundamental wave phenomenon. This article explains its physical nature using the examples of the diffraction of acoustic waves at soft and hard half-planes and at large apertures on a black screen. It is shown that the shadow radiation by opaque screens plays a central role in these diffraction phenomena. Fresnel-Kirchhoff diffraction at large apertures is presented as an asymptotic form of the shadow radiation. Fresnel and Grimaldi-type diffraction at the soft and hard half-planes is revealed as interference of the shadow radiation and the incident wave.