What E = MC^2 means, @BLACK BART is that energy and mass are equivalent, subject to a large correction (the speed of light, times itself). So how much energy resides in a tiny bit of matter? Astounding amounts (hence the amazing destructive abilities of nuclear bombs). How much matter could one form out of energy? Teeny tiny little insignificant amounts (it does go both ways, after all, and explains the electric bills of particle accelerators).
The limerick, however, addresses a fact learned from Einstein's Special Theory of Relativity, which tells us that time is a PHYSICAL property. It moves more slowly with increased gravitation and with increased speed. Two cesium clocks, at different altitudes, run slightly differently (not so as we'd notice; the difference is less than a second per million years). Which one is right - BOTH are! Because time itself is different at different gravities - and higher up there is less gravity. Likewise, time slows down with increasing speed (you have to get to more than 40% of the speed of light to notice). Your link goes to the Time Dilation equation where you can calculate the effect for yourself. So if twin brothers split up, and one goes on a space journey to Alpha Centauri and comes back in - let's say fifteen years - he is a couple of years YOUNGER than his twin upon arrival home, because time slowed down for him.
The speed of light is a theoretical limit (I am not going to go there, don't worry) but the limerick implies that if the barrier is broken, the girl in question can come home before she left. Great trick, as she can remind herself to bring whatever it was she forgot... no, wait, that gets us into time paradoxes. Let's not go there, either, at least right now.
Relativity, Special Relativity, and Quantum Physics get weird and surreal *very* quickly - and anyone who is lacksadaisical about the implications of these principles really does not understand them at all. In fact, they are SO bizarre and ridiculous that the *only* thing they have going for them is that they are undeniably factual.