This charter defines the mission, independence and objectivity, scope and responsibilities, authority, accountability and standards of the Internal Audit function.
A charter is basically a delegation / grant of authority from a person or organization of authority to one another person or organization. The purpose of an internal audit charter is to compel the departments that require to be audited to give the information needed by the auditor. Without this charter or same authority most managers wouldn't see any advantage to being audited and would likely refuse to provide anything the auditor required.