A safe stands for what is valued and guarded — secrets, memories, resources. Dream books read a locked one as protected feelings and an open one as trust or vulnerability.
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.