It is new in v11, but I do not have much detailed information about it.
When you create or edit a backup job, you can enable High priority.
The idea is to use this option to make clear this is an important job. So it can be started before less important jobs are started. It is not about job performance, it is about start time. Makes perfect sense to me! Under certain circumstances this feature will make job scheduling easier.
At first this seems to be available for Backup and Replication jobs.