Rashi citing Targum Yonasan: The key to the Beis ha'Mikdash, and the authority of Beis David.
2.
Radak: This is a metaphor. He will be the advisor of Melech Yehudah, and he will decide to close and open.
3.
Malbim: He was appointed over Beis Hashem and Beis ha'Melech.
2)
Why does it say "Al Shichmo"? People do not carry a key on the shoulder!
1.
Radak: This is a second metaphor. The key represents that he will open and lock matters of the palace. Al Shichmo is a metaphor for bearing a load. He will bear matters of the kingdom.
2.
Malbim: Officers appointed over Beis ha'Melech carry the key on their shoulder for a sign.
3)
What is the significance that he opens, and no one closes?
1.
Malbim: No one closes without his approval. This is like "Lo Yarim Ish Es Yado" (Bereishis 41:44).
