"Per dien" is Latin for "per day" or "by the day." It means that you are not guaranteed a set number of shifts to work. If the hospital needs your help on a particular day, then you may be asked to work that day. If they don't need your help, you don't work. Usually, per diem employees are simply paid an hourly wage and do not get benefits such as health insurance, paid vacation time, etc. (However, there are sometimes exceptions to that.)
"Per diem" is an employment status for legal and financial purposes. Being per diem doesn't dictate a particular type of work schedule. Each hospital decides for itself how to schedule its per diem employees. In fact, the scheduling may vary between units in the same hospital.
In most places, the per diem employees "sign up" for shifts that they want to work -- within whatever guidelines the employer has established for a minimum number of shifts per month, a minimal number of weekends, holidays, night shifts, etc. Once that schedule is made, then the employer decides on a day-to-day basis whether it needs the per diem employees who signed up for that day to come into work or not. If it's really busy, then everybody who signed up for that shift gets to work (and gets paid) -- but if it is not busy, they may tell the per diem employees that they are not needed and that should stay home (and not get paid).
Per diem employment can be a great option for someone who is seeking part time employment -- but who can tolerate the fact that they might not get to actually work all the shifts they signed up for -- or that the particular shifts they want to work might not need the help. If you are working in a department that is almost always busy and that usually has a few staff vacancies, per diem staff members will probably gets lots of opportunity to work. However, if the deparment is not terribly busy and/or they usually have plenty of staff members and rarely need extra help, the per diem staff may find that they don't get to work as often as they might like. So, you need to be prepared for all possibilities.