The words "darkness" and "gloom" share similar meanings and emphasize that the darkness will be very dark. Both words refer to a time of disaster or divine judgment. Alternate translation: "day that is full of darkness" or "day of terrible judgment" (See: [[rc://en/ta/man/translate/figs-doublet]] and [[rc://en/ta/man/translate/figs-metonymy]])
This phrase means the same thing as, and intensifies the idea of the previous phrase. Like that phrase, both "clouds" and "thick darkness" refer to divine judgment. Alternate translation: "day full of dark storm clouds" (See: [[rc://en/ta/man/translate/figs-doublet]] and [[rc://en/ta/man/translate/figs-metonymy]])
When direct sunlight shines on a mountain at dawn, it begins at the top and spreads to the bottom. When an army comes over a mountain range, it comes over the top and spreads out as it comes down. Alternate translation: "A large, mighty army comes over the mountains into the land. They spread over the land like the light from the rising sun" (See: [[rc://en/ta/man/translate/figs-simile]])
The words "large" and "mighty" share similar meanings here and emphasize that the army will be very strong. The word "army" could possibly be 1) a metaphor for a swarm of locusts or 2) referring to a human army. (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-doublet]])