What does Daniel 7:1 mean?

Back when Belshazzar had just started his reign as king of Babylon, Daniel had a dream one night. He saw these wild visions in his head, and as soon as he woke up, he wrote down everything he remembered. Daniel 7:1 - Modern Text Bible

(This sets the scene: Daniel is living in exile under Babylonian rule. He has a vivid, unsettling dream and makes sure to record it, knowing it means something important.)