Graduate seminar may refer to the doctoral coursework component, usually in the first year and maybe the second year too at some universities. In such cases it refers to different subjects such as e.g. Anthropology 8000, Social Policy 8001
It may also refer to the mandatory general weekly/monthly meetings for all research students run by your department (e.g. History department) for all research students e.g. MPhil, EdD etc in your field.
The word seminar distinguishes it from a lecture and tutorial. It means it is a bit like a traditional lecture and tutorial combined.
So you need to check. The internal departmental seminars are less common e.g. monthly and focuses on research methods and thesis writing issues.
The other seminars, if this is what it refers to, are more regular and involve assessments, exams related to different subject areas.
Annie