What do those +/- mean if linux job in background finishes (started with &)

From man bash:

In output pertaining to jobs (e.g., the output of the jobs command), the current job is always flagged with a +, and the previous job with a -.

That is, the job flagged with a + is the one that was sent to the background last.

It is also the one that will be brought into the foreground when fg is used without arguments:

$ /tmp/script &
[1] 9871
$ /tmp/script2 &
[2] 9876
$ /tmp/script3 &
[3] 9881
$ /tmp/script4 &
[4] 9886
$ jobs
[1]   Running                 /tmp/script &
[2]   Running                 /tmp/script2 &
[3]-  Running                 /tmp/script3 &
[4]+  Running                 /tmp/script4 &
$ fg
/tmp/script4

The job flagged with a - was sent to the background second last. Other background jobs are not flagged.