Chaff is what blows away when grain is winnowed. Dream tradition reads it as separating the worthless from the valuable and letting go of what has no substance.
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.