What does Genesis 1:21 mean?
In the prior verse, God spoke the command for the empty seas to fill up with living creatures, and for the empty skies to be filled with flying birds. Now in this verse, He executes that command. This is the blueprint for all six of the creative days of Genesis. God speaks and His words become reality. Later verses will continue the pattern of God observing His creation and calling it "good."The verse specifically mentions the great sea creatures or sea "monsters." As with the sun and moon, these ancient beasts of the sea were frequently worshiped as gods by ancient cultures. The Bible's claim is that those creatures were merely created things, along with all the other fish and the birds. They were made by the one, true God in a day. He alone is worthy of worship.
This verse also specifies that all of these brand new creatures were made "according to their kind." From the very beginning, God divided created life into categories: kinds of plants, kinds of fish, kinds of birds. Built into each creature was the ability, the necessity, to reproduce their own kind.
Again, God recognizes that what He has made is good, giving His approval to the world He is building.