You'll notice that the area is a little indented, which indicates that he is slightly dehydrated. Normally that would be bad, but they monitor his kidney function constantly to avoid problems. The diuretics he's on sometimes make him dehydrated because they are getting rid of fluid around his heart and lungs.
We can softly pet the top of his head and figure out whether he's dehydrated or not. It's much easier than checking the color of his urine or asking him if he's thirsty. Think of it like the little button on a turkey that tells you when it's done in the oven.