When you raise your units to job level 11, then fill all the equipment slots at that level, the job is considered mastered. When you master a job you get a % bonus to a few specific stats (the stats that it benefits vary by job). The job mastery bonus not only applies to the currently active job, but to all jobs on the unit, regardless of which one is currently active. So to fully maximize a unit you may want to master all of their jobs.
Because the amount of equipment required to fully master a job is significant, you will probably not want to master jobs you aren't using on a unit unless you use the unit very heavily and want to bring out their full potential. You may want to first verify that the stats you receive for the mastery of the unused jobs are of value to the active job.
If you wish to know the mastery bonus you receive for a specific job you should look the unit up in Alchemist Code Database: Units. The job mastery bonus is listed under the job information section, under Job Master Bonus.