What does Genesis 1:23 mean?

This verse completes the normal blueprint for the creation days in Genesis. In each case, God speaks a command, which then becomes real, God sees what He has created, and then considers it "good." This verse features the typical reference to "evening and morning," reflecting the Jewish concept of days beginning at sunset.

On this fifth day, God has created all sea creatures and all birds and blessed them with the ability to reproduce and populate water and air. This also follows a pattern seen in Genesis chapter 1, where the six days of creation are paired between the first three and the last three. God created light on day one, and the sun and moon on day four. He created the sky and seas on day two, and formed air and sea life on day five. This now sets the stage for day six: on day three, God created dry land. So, on the next day of creation, He will form both land animals and human beings.