People claim that Einstein said this, but there is no evidence that he did. It doesn't even sound like something he would say. It sounds like someone trying to be profound and wise, but in reality the quote doesn't mean anything at all. It's not philosophy OR physics, it's just pseudo-profundity. So it won't be much help to you as an English exercise, in my opinion.

Einstein has some very interesting things, but is this one of them? It seems not, according to these sites: https://quoteinvestigator.com/2012/05/16/everything-energy/ https://www.quora.com/What-did-Einstein-mean-when-he-said-Everything-is-energy-and-that%E2%80%99s-all-there-is-to-it-Match-the-frequency-of-the-reality-you-want-and-you-cannot-help-but-get-that-reality-It-can-be-no-other-way-This-is-not-philosophy-This-is-physics