A cairn is a pile of stones left by those who went before, and dream books link it to guidance, memory and marking the way. Adding a stone of your own is read as leaving a sign for others.
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.