Sackcloth is traditionally read as penance, humility and regret worn openly. Dream books see it as a wish to make amends, or as being harder on yourself than the moment requires.
When this appeared in your dream, what were you feeling?
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.