Per diem CNA

So I started my new job as a Per diem CNA at a hospital. I just had a question about the scheduling of a per diem. They told me to give them the days I am able to work and that they would call me that day if they do not need me. So far the days I have given them, they have not called me out to not work, so i have been working the days I requested for.

I also have another part time job on the side that gives me permanent days every week, so i get the schedule in advance. I am thinking about quitting this job but I do not want to quit if I will not get hours at the hospital if they do not need me.

What should I do? Or if someone could explain a per diem better as if they are one themself. thank you :)