In most books like in the picture given below, the uncertainties regarding quantum states like coherent and squeezed states are represented in phase space plot by some area enclosed within a circle or as an ellipse depending on the squeezed quadrature.
While the representation is intuitive, I do not understand the precise framework that allows to plot uncertainties. For example, what does the position or momentum axis represent? how can I plot this in a precise way or is it only for a representational purpose? I had a brief look on notions like Wigner quasi probability distribution in phase space but I fail to see how it is related and it looks quite advanced to my level at the moment. It will be very much appreciated if someone could either explain what it is or perhaps direct me to any references that cover this.