A rainbow normally represents harmony, but why does it? When we are kids we are told that at the end of the rainbow there is a leprechaun with a cauldron full of gold, and this is all obviously fake, and yet it still feels like its giving the rainbow a bit of a magical, harmonious touch. When you see the colors of the rainbow, you immediately notice they are bright and colorful, instead of black or brown, they represent something happy and harmony is exactly that.
A symbol can be used in a conversation where subtext is being implied, by using words or imagery that serve as symbols and carefully integrating them into a normal conversation, the author can make subtext work with them.