Per diem is a phrase usually used to describe something you get on a daily basis. If you are getting paid per diem, you are getting paid a certain wage for the day.
If you're lucky enough to get a per diem allowance at your job, you probably have quite a good job. It means a daily allowance for expenses. From the Latin words per meaning "by the" and diem meaning "day,"per diem can also be used, generally in a business sense, to mean simply "on a daily basis" rather than referring to an hourly rate or to an annual salary.