A rainbow is a multicolored arc caused by an optical phenomenon that, through the refraction of sunlight on drops of water suspended in the air, forms a spectrum with different colors. Because of this, the rainbow appears, generally, after the rain.
In the Bible, the rainbow is also called "bow of the covenant" or "bow of God." In Genesis, 9: 8-15, it is told that, after the flood, God, Noah and all the surviving beings of the ark formed an alliance thanks to which there would be no more floods on Earth, and that the rainbow that would appear in the heaven would be the symbol that God would use to remind them of that covenant.
God says: "This is the sign of the covenant that I establish between you and me, and with every living being that is with you, for everlasting generations: I put my bow in the clouds, which will serve as a sign of the covenant between the earth and When I make the earth cloud, the bow will appear in the clouds, and I will remember the covenant between you and me, and with every living thing, with all flesh, and the waters will no longer be a flood that destroys all flesh. "(Genesis, 9: 12-15).