"The Rainbow," by Christina Rossetti, is a short poem about the beauty of the sky and connection to the heavens through a beautiful rainbow. The speaker says the two prettiest things are clouds and rainbows.
In the first stanza, the speaker mentions boats and ships sailing on bodies of water but states the clouds, "are prettier than these."
In the next and final stanza, the speaker talks about bridges over the water. They remark bridges could be considered pretty to some, but the bridging of a rainbow, "from earth to sky is prettier far than these." The rainbow is never mentioned in the poem, but it's an implied connection based on the title. A rainbow creates the appearance of a bridge (earth to sky) in the same way a physical bridge connects two landmasses.
The bridging between earth and sky could also be a metaphorical connection to heaven based on the quote in stanza one. It could be conceived that rainbows are so important because they connect us to the heavens. The same could be said for the clouds.