What does "quem can combine the function of antecedent and relative [...]" mean?

Question

I am reading a grammar book on relative pronouns.

When talking about quem it says the following,

In addition, quem can combine the function of antecedent and relative with the meaning of
‘someone/anyone who . . .’, ‘he/those who . . .’, ‘a person/people who . . .’:

And gives two examples,

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.
She has quite a lot of stamina for someone who doesn’t exercise.

I'm not very good at grammar and don't really understand when the book says, quem can combine the function of antecedent and relative Is anyone able to explain this?

Answer 1

I think you've misinterpreted what the book says. You ask

quem can combine the function of antecedent and relative Is anyone able to explain this?

But that's not the sentence you quoted earlier. There, you have quem can combine the function of antecedent and relative with the meaning of... In other words, besides functioning as a relative pronoun with an antecedent, it has this other function. This other usage occurs in what's called a free relative clause or relative clause without an antecedent. In a "normal" relative clause, quem has an antecedent:

Os turistas a quem dei direções perderam-se na floresta.
The tourists to whom I gave directions got lost in the forest.

In this sentence, the antecedent for quem is os turistas. Rephrasing, we substitute quem for os turistas:

Dei direções a uns turistas. Estes turistas perderam-se na floresta.
I gave directions to some tourists. They got lost in the forest.

There is no such antecedent in the free relatives:

Quem segue [=As pessoas que seguem] as minhas direções perde-se.
Those who follow my directions get lost. / He who follows my directions gets lost.

Note that now in free relatives quem doesn't need to follow a preposition.

Finally, quem can also be an interrogative pronoun (where it also doesn't need to follow a preposition, though in these examples it does):

Com quem (é que) falaste?
To whom did you talk?

This is a direct interrogative. You also have indirect interrogatives:

Não sei com quem (é que) falaste.
I don't know to whom you talked.

Answer 2

Terminology

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.

Explanation?

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.