You hear this a lot in nursing as far as orders as well as employment.
PRN is an abbreviation meaning “when necessary” or “as needed” (from the Latin “pro re nata”, for an occasion that has arisen, as circumstances require, as needed).
When seen in orders, it is for an unscheduled medicine or order, basically on standby until it is needed. For example, many symptom medications such as pain, nausea, itching, sleep, and anxiety medications are designated as PRN meds. A PRN med should always have a reason attached to the order. So if you are giving morphine PRN, it should read [dose] [route] morphine [frequency] PRN for pain.
PRN can also be used in other ways as you mentioned, such as for employment status. Separately from a full or part-time employee, a PRN employee only works as needed, and can usually fill gaps in a schedule or work a varied amount of hours per any given schedule.