1)
What is the meaning of "Min ha'Naveh"?
1.
Rashi, Targum Yonasan: It is the shepherd's pen [where he tends the flock]. This is like "Nevas Keros Ro'im" (Tzefanyah 2:6).
2)
Why did Hashem say that He took David from the flock, and made him a Nagid?
1.
Malbim: You are not a king's son, and I only made you Nagid (leader). I did not yet promise you lasting kingship.