In Greek mythology, the rainbow was considered to be a path made by a messenger (Iris) between Earth and Heaven. In Chinese mythology, the rainbow was a slit in the sky sealed by Goddess Nüwa using stones of five different colors. The Irish leprechaun's secret hiding place for his crock of gold is usually said to be at the end of the rainbow (which is impossible to reach). In the Hebrew Bible, the rainbow is a symbol of the covenant between God and man, and God's promise to Noah that He would never again flood the entire Earth.

Information source:  Wikipedia