What does Genesis 1:20 mean?
Verses 20 through 23 describe God's fifth day of creation, in which He creates all the creatures of the seas, as well as the birds. This is according to the pattern of Genesis, where God used the first three days to prepare an environment, and the second three days to populate that environment. On day one God created light, and on day four He created the sun, moon, and stars. On day two God created the sea and sky, and here, on day five, is His creation of sea and air life.The pattern will then continue in later verses: On day three, God created dry land, and we will see the origin of land animals on day six.
God's spoken command is to let the waters—the seas—teem or swarm with swarms of living creatures. The picture is of filling up an ocean intended to carry the abundance of God's creation. In a similar way, God commands the sky to be full of birds that would fly across its previously empty expanse. This is all part of a process where God's design comes into focus. He didn't merely create the oceans for their own sake. The purpose of the seas was to support active, teeming life. The purpose of the empty sky was to host the countless variety of birds He would create.