Here "darkness" is a metaphor for not knowing the truth about God. And "light" is a metaphor for God's true message that saves people from their sin. (See: [[rc://en/ta/man/jit/figs-metaphor]])
These words can be combined with the sentence beginning with "The land of Zebulun" (verse 15). Alternate translation: "In the territory of Zebulun and Naphtali ... where many Gentiles live, the people who sat"
"who were living ... who were living." The word "sat" is an idiom for having lived for a long time in one place, not to sitting on the ground or a piece of furniture. (See: [[rc://en/ta/man/jit/figs-idiom]])
This basically has the same meaning as the first part of the sentence. Here "those who sat in the region and shadow of death" is a metaphor. It represents those who did not know God. These people were in danger of dying and being separated from God forever. (See: [[rc://en/ta/man/jit/figs-parallelism]] and [[rc://en/ta/man/jit/figs-metaphor]])