A lullaby is read as comfort and safety, often reaching back to childhood. Dream books link it to the wish to be soothed, or to soothe someone who is in your care.
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.