“the last days.” In this context, the “last days” are the Millennial Kingdom, although at the time Isaiah was writing God had not revealed that there would be a 1,000-year Millennial Kingdom followed by an Eternal Kingdom with a gigantic city from heaven (Rev. 21, 22). Thus Isaish thought of the “last days” as the Messianic Age, the kingdom of Christ on earth. [For more on the coming kingdom of Christ on earth, see Appendix 3, “Christ’s Future Kingdom on Earth”].
“chief.” Not just the highest, but the most important, because the Temple will be on the top, and the city of Jerusalem where Christ will have his palace will be on the south side of the Temple. The Jerusalem and the palace of Christ will be on the south side of Mount Zion is significant because in the biblical culture, “east” was straight ahead and so south was on the right and north was on the left. For Christ to be “on the right hand” of God as the prophecies state, he will have to live and reign south of the Temple where God dwells. Mount Zion will be the highest mountain in the world (Isa. 2:2; Micah 4:1; Ezek. 20:40).
“all nations.” In the future, all nations will come to Jerusalem to worship and to be blessed (Ps. 86:9; Isa. 56:6-8; Jer. 3:17; 16:19).
“stream.” The Hebrew word translated “stream” is nahar (#05102 נָהַר), to flow or stream, and it is related to the Hebrew word for “river.” When God is in His Temple in Jerusalem and Christ is reigning as king, the nations won’t just “trickle” into Jerusalem, they will come as a river of people.