1)
What does this Mizmor discuss?
1.
Radak: It is when Sha'ul went to look for David in Tzurei ha'Ye'elim (Shmuel I, 24:3), and David hid in a cave. David composed this at the time.
2)
Which cave is this?
1.
Rashi: It is the cave in which David cut the corner of Sha'ul's garment.
