The function f(x,y) = … is a gnuplot trick to color the two sides of a classification boundary red and white. The 1/0 results in an “error”, forcing gnuplot to ignore the color. f(x, y) uses this trick to plot out the region of the relu where the output would be > x, showing the classification boundary created by a single relu. Once you superimpose multiple such regions, you get to see the closed region which is the classification boundary of all the relus combined.