Source: Willard Richards Pocket Companion
The Words of Joseph Smith: Page 11
What does Joseph Smith mean by the building up of Zion?
For Latter-day Saints (Mormons), there are a couple of different meanings to the word Zion. Zion is the pure in heart who are of one mind with God and live in righteousness (see D&C 97:21 and Moses 7:18). Zion also means a place or society where the pure in heart live. The city built by Enoch, for example, which was eventually taken to heaven was named Zion (D&C 38:4; Moses 7:18–21) and the future city which we believe will be built near Jackson County, Missouri, USA is also referred to as the city of Zion (D&C 103:11–22; 133:18).
So when Joseph Smith says the building up of Zion should be our greatest object, which Zion is he referring to 1) Zion, the righteous individual, or 2) Zion, the city and society? I believe the answer is both. The two types of Zion are very inter-related in that the city of Zion will be populated with Zion individuals. There would be little point in building up the city of Zion if you do not have Zion people to live there.
You actually see, in the context of the speech as a whole, that Joseph Smith is talking about building up a Zion people as well as a Zion society. The speech has much discussion about receiving personal Revelation, the ministering of Angels, the work and authority of the Priesthood, and how to return to the Presence of God all of which are necessary for individuals to become Zion-like. But he also spoke much about Preaching the Gospel, the Gathering of Israel, Temple Work, and the Second Coming, all of which are key ingredients of the Zion society.