Definition: A "staff sergeant" is a military rank. It's a noncommissioned officer, which means this person is not an officer (who usually has a higher level of authority) but still has important responsibilities. In the army or marines, a staff sergeant ranks above a corporal and below a sergeant first class. In the air force, they rank above an airman first class.
In civilian life, "staff" can refer to a group of employees in a company, while "sergeant" can refer to a police rank in law enforcement, but these uses are not related to each other.
A staff sergeant is an important role in the military, acting as a leader and trainer for soldiers.