This article introduces the fundamental principles governing zonal flows for researchers who are engaged in experiments. The mechanism of the Geodesic Acoustic Mode (GAM) is first introduced for a simple tokamak having a circular cross section. The theoretical basis of which was described a rather long time ago. The theory is then extended to include helical systems based on the perspective that the geodesic curvature of the confining magnetic field plays an essential role. It is found that a short connection length in helical systems lends GAM oscillations different features from those in tokamaks. Special attention is paid to the low frequency range and a new branch is analyzed, which is referred to in this article as low frequency GAM. The zonal flow in the lowest frequency range is known as stationary zonal flow and known to have weak damping. Finally, an attempt is made to unify the theories of these two different frequency ranges.