Both of these phrases mean the same thing and are combined for emphasis. They refer to "spiritual darkness." This means all the other people of the world will not know Yahweh or how to please him. This is a metaphor for divine judgment. (See: [[rc://en/ta/man/translate/figs-parallelism]] and [[rc://en/ta/man/translate/figs-metaphor]])
This means the light of God's presence will appear for the people of Israel, and it will show the way they should go. (See: [[rc://en/ta/man/translate/figs-metaphor]])