Joel says “a fountain shall come forth from the house of the LORD” and water the Valley of Shittim. This is part of the closing picture of renewed life in the land. See Joel 3:18.
In Joel 3:17–21, the speaker promises that Judah will recognize Yahweh as their God because he is present in Zion, making Jerusalem set apart and protected from hostile intruders. The vision then shifts to abundance: the land overflows with wine, milk, and water, even sending a life-giving stream outward from Yahweh’s house. These verses close Joel’s final movement, following the gathering of nations for judgment and the cosmic signs that accompany Yahweh’s decisive intervention (earlier in chapter 3).
These closing verses describe a future outcome after God’s decisive intervention: Judah comes to recognize that Yahweh is their God because he is present in Zion (verse 17). Jerusalem is portrayed as set apart (“holy”) and secure from the kind of “strangers” who previously moved through it as a threat (verse 17).
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with Joel 3:18, then read the surrounding section so the answer stays connected to the passage's flow.
18It will happen in that day, That the mountains will drop down sweet wine, The hills will flow with milk, All the brooks of Judah will flow with waters; And a fountain will come forth from the house of Yahweh, And will water the valley of Shittim.