What does Genesis 1:17 mean?

This verse repeats statements made in prior verses. This is meant to fit the structured, poetic Hebrew language structure used in verses 14 through 18. This structure is sometimes called a chiasm. This is a specific way of arranging the words to emphasize the central idea of what is being described. In this case, for all the details which this passage does not include, the central point becomes all the more clear: heavenly bodies are not gods, they are created by the One True God.

The central purpose of the sun, moon, and stars was to provide light on the earth. These also allow people to track time (Genesis 1:14) and inspire us to glorify God for His creative power (Psalm 19:1). People living in the modern era often have easy access to artificial light. In fact, it can be nearly impossible to escape for those living in large cities. As a result, it's hard to appreciate how powerful and necessary and beautiful natural light truly is. We can easily take for granted this cornerstone of God's creation.