It’s an illustration of the 3-adic integers. I thought this video gave a nice explanation of p-adic numbers.

The Riemann surface of the multi-valued square root function on the complex numbers. The original image is here.

Fair point, and of course Grothendieck was notoriously extreme on this point (retiring completely from society and asking for his works to be destroyed). For us modern readers, it’s probably more interesting to view this as a glimpse into his mind and the mind of the mathematical community at the time than any kind of guidance.

What do you find misanthropic about it? If anything, I feel like he’s embracing the human side of research.