1)
What does this Mizmor discuss?
1.
Radak: David captured it when he was exiled, fleeing Sha'ul.
2)
Why does it say "Tefilah l'David"?
1.
Radak: He prays that Hashem guard and save him from those who seek to kill him.
3)
Why does it say that David was poor?
1.
Radak #1: Where he went, he did not have his needs, unless his relatives or people of Yehudah helped him covertly. Or, when he was with Achish, he financed himself from spoils of other nations.
2.
Radak #2: He was like an Oni who has no helpers. People of Yisrael did not help him, for Sha'ul chased him without reason.