The question Camus is asking is whether life is worth living, which, he believes, is equivalent to the problem of suicide. For the equivalence, Camus establishes such auxiliary assumptions as living is absurd and man should act upon his belief. Camus says there are more than two immediate answers: Yes and No. A third possible answer, according to Camus, is, "Yes to mean in one way or the other." To explain how this third type answer is possible, Camus appeals to the Nietzschean criterion. So what is the criterion?
Nietzsche himself does not offer a criterion, thus we must infer the criterion from the work of Nietzsche. Nietzsche believes that there are two kinds of people: those who believe in God (or common pejorative-sense morality: cf "Nietzsche's Moral and Political Philosophy" in SEP) and supermen (Übermensch: human perfection). God is the guarantor of meaning for Christians. According to Nietzsche, those who believe in God think that life after death is the reality and thus has the true meaning. But they also believe that the life on earth also has meaning since it is the creation by the God.
By the Nietzschean criterion, thus, Camus must be meaning these two types of people: Christians and supermen. And by they, Camus must mean the Christians. Viewed in this light,
they think "yes" in one way or the other
should mean that Christians think that life (on earth) is worth living and life in after-life (heaven) is also worth living.
Appendix (Hopefully this appendix has some use value, unlike the one in human body whose only function is to blow up)
- A helpful block quote of Nietzsche's influence on Camus which relates to my answer. https://plato.stanford.edu/entries/camus/#NupCamStaPoi
Camus is sometimes mistakenly called a “pagan” because he rejects
Christianity as based on a hope for a life beyond this life. Hope is
the error Camus wishes to avoid. Rejecting “the delusions of hope” (N,
74), Nuptials contains an evocation of an alternative. Camus relies
for this line of thought on Nietzsche’s discussion of Pandora’s Box in
Human, All Too Human: all the evils of humankind, including plagues
and disease, have been let loose on the world by Zeus, but the
remaining evil, hope, is kept hidden away in the box and treasured.
But why, we may ask, is hope an evil? Nietzsche explains that humans
have come to see hope as their greatest good, while Zeus, knowing
better, has meant it as the greatest source of trouble. It is, after
all, the reason why humans let themselves be tormented—because they
anticipate an ultimate reward (Nietzsche 1878/1996, 58). For Camus,
following this reading of Nietzsche closely, the conventional solution
is in fact the problem: hope is disastrous for humans inasmuch as it
leads them to minimize the value of this life except as preparation
for a life beyond.
- With the superman idea, according to the following SEP article, what Nietzsche has in mind is to understand what kind of life is a well-lived life. To Nietzsche, of course, the life of the superman, not that of a Christian. Why? Some key concepts to answer the question are power, perfection and eternal recurrence. The author of the article, however, concludes that there is no shared answer on this point. The author, for instance, considers the following two proposals, and rejects them:
This leaves the question whether there are (formal or substantive) criteria of “perfection” for Nietzsche? Many writers (e.g., Hurka 2007; Nehamas 1985; Richardson 1996) are attracted to the idea that “style” or “unity” is a criterion of excellence or perfection for Nietzsche, and, indeed, as noted above, the pursuit of a unified or coherent life project is a characteristic feature of those Nietzsche deems to be higher men. Whether such style or coherence suffices is a vexed interpretive question, since it is not entirely clear that the formal criterion of style or unity is available only to Goethes and Beethovens: did not Kant, that “catastrophic spider” as Nietzsche unflatteringly calls him (A 11), exhibit an extraordinarily coherent style of creative productivity over many years?
Others (e.g., Magnus 1978) take Nietzsche's idea of eternal recurrence (the hallmark of life-affirmation, as noted above) as the criterion of a well-lived life: perfection is a matter of living in such a way that one is ready to gladly will the repetition of one's life, in all its particulars, in to eternity. This, too, seems both too thin and too severe as a criterion of perfection standing alone: too thin, because anyone suitably superficial and complacent might will the eternal return; too severe, because it seems to require that a post-Holocaust Goethe gladly will the repetition of the Holocaust.