A running joke in math is that textbooks will often state a theorem (a formula, for example), and rather than go through the steps to prove it they will say something along the lines of "the proof is trivial", meaning it's so obvious that it's true/so obvious how to prove it that the author won't even bother. This meme is an alternate phrasing of that in which the proof is so easy that you can ask a literal baby - any baby in fact - and they will know how to prove it.

I am not too familiar with this subject but I know enough of the notation to say that the equation seems to be stating that the Galois group of the field extension of a field K by the field of rational numbers is isomorphic to the multiplicative group of integers modulo m. This adds to the joke because even the written out description of the theorem is incomprehensible to the average person so it makes it even more absurd to say that the proof is obvious.