(Mishnah): A Sukah that is 'Medubleles" (the Gemara will explain) that has more shade than sun is valid.

If the Schach is as thick as the roof of a house, even if the stars are not visible through the Schach, it is valid.

(Gemara): Rav and Shmuel disagreed over the understanding of the word 'Medubleles" -

One opinion said that it is 'poor', i.e. its Schach is scarce.

The other opinion said that it is 'mixed', i.e. uneven, thereby causing there be more sun than shade, even though if the Schach would be lined up, there would be more shade than sun (like this ).

The opinion that says that it is 'poor', the shade must still be more than the sun (and the two phrases of the Mishnah are discussing one case).

The opinion that says that it is 'mixed' say that when it is mixed, the shade is not more than the sun (and the two phrases of the Mishnah are two different cases).

(The Mishnah said that if the stars are not visible through the Schach, it is valid) This shows that ideally, the stars must be visible.

R. Levi quoting R. Chamah bar Chaninah: The Mishnah is referring to the 'stars', i.e. the rays of the sun. If they are not visible, it is valid, but not ideal; but even ideally, the stars at night need not be visible.