I’m glad I’m not the only one who noticed this.
tl;dr version, it stems from a discussion involving ancient beliefs that the world was supported on the back of a great turtle. Someone asked what, then, supported the turtle, to which someone else responded “It’s turtles all the way down, kid.”

In case it’s early and you don’t want to brain that, It shows the fallacy inherent in many belief systems that the impossible is explained through some sort of phenomenon which is itself unexplained.