Please refer the below link to get information about thread.
Single thread = 1 job at a time. This is usually for jobs that share things like files, dataareas and dataqs. This way you can prevent data form changing by another job running at the same time.
Mult-threaded = Many jobs can run concurrently. This is controlled by the MAXACT parameter used to create the JOBQ. This would be common for jobs like reporting, or inquiry. Mainly where files can be used my many jobs and still maintain their integrity from job to job.