This service ensures that a scheduled job can only run on one node of a cluster at a time. A scheduled job could be, for example, an Activities feed job that generates an email to send to everyone every night, or a content cleaner job that cleans up orphaned content.
The JobLockService is used to provide a locking service at the job level, rather than the node level. It’s for example used indirectly via the AbstractScheduledLockedJob QuarzJobBean.
For an example of using the JobLockService see the Content Store Cleaner code on GitHub.
See also Scheduled Jobs.