In High Energy Physics papers, I often see a diagram as shown below:

I am still studying Quantum Field Theory, and I could not understand the only reference I could find about this plot: https://arxiv.org/abs/1702.00330. What I know for sure is that the y-axis is strange quark mass, x-axis is light quark (u and d) mass. And the third axis coming out of the plane is chemical potential ($\mu = 0$).

1. What are the various features of this plot (in a simpler language)?
2. Why are masses taken as variables when we know their values to be constants?
3. What is the significance of this plot?
4. How is it related to QCD phase diagram?