Dear Susanta,
Your query is a little surprising. When employees of the other departments are not clear about their role, they approach the HR department to clarify that. Now you being from HR asking this query is a little bit preposterous.
When you joined the company, did the selectors give you Job Description (JD)? If yes, start working on it. Secondly, you must have worked in the HR Department at the junior level. Have you observed your Manager working? Considering your past experience, identify the gaps on your own.
Lastly, we the members of this forum are outsiders and we can tell a hundred things. But these may not matter to your top boss. Therefore, you better approach your reporting authority and understand what will be your deliverables, how your performance will be measured etc. Your survival in the company will depend on the fulfilment of the expectations of your boss or even the boss's boss.
Thanks,
Dinesh Divekar