Why did David compose this Mizmor?
Radak, Malbim: It was for when they will bring the Aron into Kodesh ha'Kodoshim. This was after David offered an Olah on Har ha'Moriyah, and the place of Beis Hashem was revealed to him (Divrei ha'Yamim I, 22:1).
What are "ha'Aretz" and "Tevel"?
Rashi: They are Eretz Yisrael and other lands, respectively. Malbim - Hashem designated Eretz Yisrael for special Hashgachah. From the creation, He desired a dwelling below, but people's sins blocked this. The Avos drew Hashgachah and the Shechinah to the ground, until it dwelled in the Mishkan 1 .
Radak: They are the entire land (Malbim - the entire earth) and the settled part, respectively. This is why it says "u'Melo'ah" regarding ha'Aretz, and "Yoshvei Vah" regarding Tevel.
Malbim: A parable for this is a king who decreed that Romans not go to Surya, or vice-versa. Later, he wanted to marry a Syrian woman; he nullified the decree. So Hashem said "ha'Shamayim Shamayim la'Shem veha'Aretz Nasan li'Vnei Adam" (115:16). When He came to give Torah to Yisrael, He nullified this. He descended, and told Moshe to ascend. (Shemos Rabah 12:3 and Pesikta d'Rav Kahana 12 say similarly.)
