A cellar door leads down to what is stored and half forgotten. Dream books read it as the entrance to buried memories or instincts, and opening it as readiness to look at them.
Has this come up in your dreams before? What was going on in your life then?
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.