en_tn/jer/04/27.md

land will mourn, and the heavens above will darken

Jeremiah emphasizes Yahweh's judgment by saying that the earth itself expresses great sorrow. (See:rc://en/ta/man/translate/figs-personification)

cavalry

a group of soldiers riding horses

every city will flee... every city will climb

This emphasizes that all of the people will be running away.

The cities will be abandoned, for there will be no one to inhabit them

These two clauses have similar meanings. The second one strengthens the thought in the first. (See” Parallelism) AT: "There will be no one left in the cities."