An echo is your own voice coming back, so dream books read it as reflection, repetition and words that return to you. It can also stand for the past still sounding in the present.
Where in your waking life have you met something like this lately?
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.