A cascade of water falls straight down for thousands of feet, suspected to be someone vomiting wildly.
(Note: This is a humorous and literal translation. The original Chinese lines are a playful parody of Li Bai's famous poem, substituting the second line for a comical effect.)
If you'd like a more poetic translation that preserves the humor, it could be:
"A waterfall plunges three thousand feet, as if someone’s stomach can’t compete."