The Gauss beam describes the shape of the incoming light at pixel level
The graph allows you to learn, how the Depth of Focus , so to say the image side DOF, is influenced by the beam waist diameter (= Airy Disk) , the Wavelength and the Circle of Confusion (CoC).
The Airy Disk describes the resolution of the lens.
For given CoC the Depth of Focus is maximized for an Airy Disk that is COC/sqrt(2).
The Depth of focus also depends inverse linearly on the Wavelength.
Double wavelength = half Depth of Focus.