What is this type of diagram called and how to interpret?
They used a stain that marks tyrosine hydroyxylase (most likely using an antibody to TH, followed by a secondary antibody tagged to an enzyme that then produces a dark precipitate in the presence of the proper precursor).

The dark regions are where more antibody bound, so you can infer this is where TH is highest. It's very difficult using these techniques to take anything quantitative from this, but you can use the technique to identify regions and possibly compare between them, given you have an appropriate control region.