As a smooth brain, I needed the explanation:
In this comic, we find Einstein imagining the scenario that would later help to make him famous, but before any particularly profound revelations have been established. It is currently just, so to speak, a flight of fantasy.
In a similar vein, the Title text refers to one of the long-standing issues about the orbit of Mercury - that it doesn't quite orbit the Sun in the way that Newtonian physics would suggest. We now know that this is accounted for by General Relativity
ngl, not my favourite xkcd, but I did learn something. Maybe a few things.