Shell background processes: what do characters in termination message mean?

The + means that it is the most recent background process. This is the one that will become the foreground process if you type fg.

I don't know about the - but I suspect it the one prior to the most recent one. This would be the one that would become the + if you kill the most recent one.

There may also be a current/old system the way cd - works.

Ah yes, the manual backs me up on both points.

You can use fg + and fg - to bring these jobs to the foreground.

You can also get a list of all your currently running jobs and their statuses with the jobs command.

plus sign means the last job you suspended, so fg alone will recall it to the foreground. minus sign means the job before the last one that you suspended. what you see agrees with this designation.