Joel says the LORD roars from Zion and utters his voice from Jerusalem, and the heavens and the earth quake. He also says the LORD is a refuge and stronghold for his people. See Joel 3:16.
Joel pictures a crowded “valley of decision” where Yahweh’s decisive day is close. The scene intensifies with cosmic disruption: sun and moon go dark and stars stop shining. Then Yahweh is portrayed as roaring from Zion and thundering from Jerusalem, shaking heaven and earth. These verses sit inside Joel’s closing vision of nations gathered for confrontation and accounting in a named valley (earlier in the chapter), leading to a climactic “day of Yahweh” scene.
These verses present the “day of Yahweh” as an imminent moment when Yahweh acts decisively in public view. The scene is crowded (“multitudes”) and located in a named “valley of decision,” stressing that the outcome is not uncertain (verse 14). The approach of this day is marked by cosmic disturbance: normal sources of light fail (verse 15).
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 3:16, then read the surrounding section so the answer stays connected to the passage's flow.
16Yahweh will roar from Zion, And thunder from Jerusalem; And the heavens and the earth will shake; But Yahweh will be a refuge to his people, And a stronghold to the children of Israel.