Yahweh continues speaking to the people of Jerusalem.
This is likely a description of the rain and snow evaporating.
Yahweh speaks of his word as if it were a person whom he sends out as his messenger to accomplish a task. (See: rc://en/ta/man/translate/figs-personification and rc://en/ta/man/translate/figs-metaphor)
Here the word "mouth" represents Yahweh himself. AT: "the word that I speak" (See: rc://en/ta/man/translate/figs-synecdoche)
Here the description of the word returning to Yahweh means that it has completed the task that Yahweh had sent it to complete. That it will not return "empty" means that it will not fail to accomplish its task. AT: "it will not fail to complete its task" (See: rc://en/ta/man/translate/figs-explicit)