bash: What is a jobspec

The job control section of Greg’s Bash Guide describes this as follows:

A job specification or “jobspec” is a way of referring to the processes that make up a job. A jobspec may be:

• %n to refer to job number n.
• %str to refer to a job which was started by a command beginning with str. It is an error if there is more than one such job.
• %?str to refer to a job which was started by a command containing str. It is an error if there is more than one such job.
• %% or %+ to refer to the current job: the one most recently started in the background, or suspended from the foreground. fg and bg will operate on this job if no jobspec is given.
• %- for the previous job (the job that was %% before the current one).