"the things that you heard or read" 所means “that”。东西means：things

It is a kind of sentence pattern: Someone 所 V 的 N Someone 所 V 的 N = The N that someone V For example: 你所說的話 = The thing/word that you said 我所吃的東西 = The thing/food that I eat Personally, I think it is a function word. It does not really carry a specific meaning, but has its function in a sentence.

Thank you!

It means 'everything' "Everything that you heard or read" Without 所, the sentence means "The things you heard or read" (Note, I'm not a native speaker but have learned for 20 years)