doing a 'man jobs' brings up the tcsh man page, which states among other things:
The shell maintains a notion of the current and previous
jobs. In output pertaining to jobs, the current job is
marked with a `+' and the previous job with a `-'. The
abbreviations `%+', `%', and (by analogy with the syntax
of the history mechanism) `%%' all refer to the current
job, and `%-' refers to the previous job.
while this is the tcsh man page, it applies to most, if not all, other standard shells, too. to be sure, check your shell.