The retina is where light becomes sight, so dream books read it as perception and the way you take in the world. It points to how clearly you register what is in front of you.
Where in your waking life have you met something like this lately?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.