Chalk is soft and easily rubbed away, so it stands for things written or decided only for a while. Dream books also link it to learning, lessons and white cliffs facing the sea.
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.