Both in English and Portuguese, some words are used to refer to another previous word. For example, the word she refers to a feminine thing that was cited:
I have a wife. She like potatoes and pizza.
In the example above, the word (pronoun) she refers to word wife. Using the terminology, the expression she is the relative and wife is the antecedent.
I think the book meant to say that quem can also instead of refer to something, be the thing that is referred to. Which looks like an edgy teenage quote. If I understood what it wanted to say by the examples, it seems like a bad way of explaining it.
In English, who either refers to a person in a question, being an interrogative pronoun, ("who is he?") or refers to a person mentioned before right after a sentence, being a relative pronoun, ("Someone who loves cheese"). In Portuguese, it can do both ("quem é ele?" and "alguém quem ama queijo"), but it can also refer to an indefinite person without referring to a previous word in the form of "someone who" or "anyone who", being an indefinite pronoun. For example,
Quem gosta de peixe vai adorar este novo restaurante.
Anyone who likes fish will love this new restaurant.
Para quem não faz exercício, ela tem bastante fôlego.
For someone who does not exercise, she has great stamina.
This usage of quem has a parallel in English in an informal or archaic usage of who:
Who eats cheese is alright by me.
Give gold to who eat cheese and death to who don't.