This rainbow signified God's grace. This is significant. Let us therefore come boldly unto the throne of grace, that we may obtain mercy, and find grace to help in time of need. This grace that Noah found is at God's throne. Its therefore no wonder that we read this about God's throne in the Book of Revelation.
Rainbows indicate no more floods.
Spiritually speaking, floods speak of death. Death cannot stop us if we find grace from God upon our lives. Find a rainbow all around it. Once Noah went through the flood, God showed him a rainbow saying there would be no more floods. When we die with Jesus Christ, we will never die again.