Here is the wiki def.
Line authority - in which individuals in management positions have the formal power to direct and control immediate subordinates.
Staff authority - granted to staff specialists in their areas of expertise. Narrower than line authority and includes the right to advise, recommend, and counsel in the staff specialists' area of expertise. It is a communication relationship with management. It has an influence that derives indirectly from line authority at a higher level.
and take a human resource manager as an instance