QPJOBLOG in AS400/iSeries speak is a spooled file created by a job containing the job log of the job when it completes.
The QPJOBLOG will normally contain information gathered during the processing of a job. Depending on the logging level set, you may or may not see all the commands and programs used by the job, various error messages and informational messages.
Joblogs for different jobs can be very different in their nature, dependent on what each specific job does. Joblog management can be a key focus in an AS400/iSeries environment.