From Wikipedia [1]
"Mathematical induction proves that we can climb as high as we like on a ladder, by proving that we can climb onto the bottom rung (the basis) and that from each rung we can climb up to the next one (the step)."
For-loops are a discrete form of mathematical induction and is a basic concept to do any kind of programming, hence the joke (of course its more nuanced than that but is true in a tongue-in-cheek kind of way)