Job tags are simple attributes which can be used to mark jobs and job slots for later filtering. You define your own tags and then optionally associate them with job definitions and slots. They can then be reported and filtered on in places which support filtering on job definitions and job slots.
To use job tags, you must first enable them on the Setup panel using the System preferences and feature enabling tool: check the box Job tags on the Feature enabling panel. Define job tags themselves with the Job tags link on the Setup panel under Jobs.
Job tags for "Jobs and job slots" can be associated with both jobs and job slots; those for "Jobs" or just "Job slots" can only be linked to those objects.
A job slot passes a job tags filter if the job slot and/or the job definition for that slot have one or more of the required job tags. Effectively this means that all job slots inherit the job tags of the jobs they're based on.
Note that job tags are global, meaning that they work within all projects and are not project-specific.