Obviously, I can't know the specifics of the job description in your institution -- but I think you know that are just looking for general possibilities. Be sure to discuss these things in your interview.
1. The #1 thing is probably that you will be expected to keep everybody happy. The staff, the physicians, the patients, the upper level administration, other hospital departments, etc. will all expect "the management" to meet their needs and wants. So, they will bring all of their problems to you and expect to have you improve their situation. This is the main reason I have never taken a straight management job.
2. Budget. It will probably be your responsibility to control the use of resources on your unit. Resources include the employees, the disposable supplies, the equipment, etc. Monitoring your consumption of resources and being cost-effective in the department's use of them.
3. Hiring -- assist with recruiting, interviewing, hiring decisions, negotiating with job applicants, etc.
4. Disciplinary action -- giving people feedback (both positive & negative), delivering reprimands, suspensions, firing, etc.
5. Assuring compliance with regulations, standards of care, etc. This is another big reason I never wanted such a job.
Oops... gotta run. I'm sure others can add to this list.