This means "as needed". I was a per diem phlebotomist for two years while I went to school. In my case it was a non benifited position (but the pay was higher as a result) and the only way to get hours was to sign up for the left over needed positions and take sick calls from other benifited employees. It is great if you want to go to school, want more money or just more flexability in your work schedule.
I would advise against it if you like a set schedule and a standard pay check... depending on the quality of needed positions left over you may be working nights, weekends and holidays...
hope that helps 😉