What does "per diem" mean in regards to salary for employment? I'm looking for a job. I need to know what a job ad means when they say the position...

The phrase "per diem" means "per day" or "by the day."

In terms of jobs, if the position itself is "per diem" it probably means that it is not steady work.  Instead, they will, at best, be hiring you on days when they need you (sort of like substitute teaching would be).

Per diem can also refer to payments that you get in addition to your regular pay.  For example, if you go off to an assignment somewhere out of town, your employer might give you a per diem for food and lodging.

But it sounds like the first definition is more applicable in your case.