The relative pronoun "that" refers to the word "aspect" and its modifiers. Approximate meaning: One crucial aspect of Einstein’s training is the years of philosophy he read as a teenager. This crucial aspect is often overlooked. BTW, " the years of philosophy he read as a teenager," strikes me as odd; in American English, I would say "the years he spent studying philosophy as a teenager."

that in this sentence means the years of reading . eg crucial aspect ... that is......the years of philosophy. http://www.thefreedictionary.com/that