Psalm 87:1, "His foundation is in the holy mountains," is a poetic statement about the security and divinely established nature of Zion (Jerusalem) and, by extension, the people of God. Let's break down the meaning:

"His foundation": "His" refers to God. This phrase implies that something is firmly and permanently established by God. The focus is on the secure and enduring nature of what is being described.

"in the holy mountains": This refers to the mountains surrounding and including Jerusalem. These mountains held religious significance:
They provided a sense of physical security and defense for the city.
Mount Zion, specifically, was the site of the Temple, the center of Israelite worship and God's presence.
"Holy" indicates these mountains were set apart for God's purpose and associated with his sacred presence.

Therefore, the verse means:

Divine Establishment: God himself has established Zion. It's not a human construct but something divinely ordained.
Security and Stability: Just as a building with a firm foundation on a mountain is stable and secure, so too is Zion. Its security comes from God.
Sacred Place: The mountains' holiness underscores that Zion is a place of God's presence, worship, and blessing.
Unshakeable Kingdom: By extension, this verse suggests that God's kingdom, centered in Zion, is unshakeable and eternal.

Figurative Interpretation

Many interpret this verse as a metaphor for:

The Church: In the New Testament, the Church is often seen as the "new Jerusalem." The verse can then be understood as meaning that the Church is founded on Christ (the "rock"), divinely established, secure, and a place of God's presence.
God's Kingdom: The verse can also refer to the broader concept of God's kingdom, which is built on his righteousness, love, and power, and is therefore enduring.

In summary, Psalm 87:1 asserts that Zion (Jerusalem), and by extension, God's kingdom and people, are divinely established, secure, and holy, with a firm foundation rooted in God himself.