I have never seen cross-hatching used in this kind of graph before (so take this with a grain of salt). However, the basic concepts are more familiar. Let us think of the solid black line as being composed of two halves, a left leg and a right leg. The solid black line is the rate or activity of the enzyme on a percent scale. The enzyme has (at least) two forms, EH and E-, with an equilibrium governed by pKa1. At high pH most of the enzyme is in the E- form, but at pH values below pKa1, the enzyme is mostly in the EH form. I think that E- is prevalent in the hatched area to the right of the left leg. The substrate also has two forms, SH+ and S, and this equilibrium is governed by pKa2. At pH values below pKa2, most of the substrate is in the SH+ form, and this corresponds to the hatched area on the left side of the right leg. Beyond that pH, S would be the predominant form. The cross-hatched area is a region in which both E- and SH+ can be found, and the model implicitly assumes that these two ionic forms are the only pair that is catalytically active. Therefore, this is where the rate (activity) is highest.