The job priority can be between 1 and 99 with 1 being the highest or most important priority. During a regular schedule run (where you have not selected an alternate form of scheduling) jobs are schedule by their priority plus due date.  Jobs with higher priority plus earlier due dates are allocated resources before lower priority jobs  with later due dates.

You may be limited by the user groups setting as to what level of priority you can set a job to.  For example, you may be limited to a "highest priority" of 25 meaning that you can set a job's priority to be from a high of 25 down to a low priority of 99.