According to Genesis, a rainbow marked God’s covenant with Noah after the Flood.
Genesis 9 describes God’s covenant with Noah, his descendants, and every living creature that had survived the Ark. The rainbow in the cloud is presented as the sign of that covenant. Its purpose was to recall the promise that a flood would not again destroy all life on earth.
The passage makes the covenant broader than a private agreement with Noah alone. It includes future generations and animals, which is why the rainbow is associated with a universal promise in the biblical story. Genesis does not describe the rainbow as something first created at that moment; it identifies it as the covenant sign.
The rainbow is sometimes confused with the dove, but the dove belongs to the earlier search for dry land. After Noah left the Ark, he built an altar and offered sacrifices; the rainbow covenant follows that event.