Its about a certain form of reinforced learning, called Temporal Difference Learning:
https://inria.hal.science/hal-00840470/document at page 11.
The calculation is called simple because it follows directly from the definitions above. Simple means more like "follows directly from definition without fancy math tricks", not " everyone who can read could solve this in 10 minutes".
I think this is also mostly math related, not physics?
I had a good laugh anyway. Good meme