This message appears in a pop-up dialog box whenever you start mailtool(1) while another mail reader has the inbox locked. A question follows: Do you wish to ask that mail reader to save the changes? You are given three choices.
If you choose Save Changes, mailtool(1) requests the other mail reader to relinquish its lock and write out any changes it has made to your inbox. If you choose Ignore, mailtool(1) reads your inbox without locking it. If you choose Cancel, mailtool(1) exits.