After Noah's Flood, God placed a rainbow in the sky as the sign of the covenant.
Genesis 9 describes a covenant between God, Noah, his descendants, and every living creature that had survived the Flood. God says the rainbow will appear in the clouds as a reminder of the promise never again to destroy all flesh by a flood of that kind.
The passage presents the rainbow as a covenant sign, not as a claim that rainbows first began to exist at that moment. Its narrative function is to mark a promise and provide a visible reminder whenever clouds gather over the earth.
The covenant is broader than Noah's immediate family: it includes future generations and living animals. This is why the rainbow is central to the post-Flood conclusion even though Noah himself is the human recipient of the covenant story. Modern uses of the rainbow can carry additional meanings, but the Genesis symbol specifically concerns this covenant.