1)
What are "Metzados"?
1.
Radak: They are tall rocks 1 in the Midbar, like the verse explains, in the mountain in Midbar Zif.
1
Malbim: (19): They are rocks.
2)
Why does it say twice "va'Yeshev"?
1.
Malbim: David's strategy was to alternate dwelling in two places - sometimes in the Midbar ba'Metzados, and sometimes in the mountain in Midbar Zif. Via this, Hashem did not hand him over to Sha'ul - whenever he sought him in one place, he was in the other.
