The secondary is responsible for:
A defensive back is another name for a player in the secondary. Defensive backs can refer to cornerbacks, linebackers, free safeties, or strong safeties.
There are special names for defenses that require extra defensive backs: five DBs is a "nickel" scheme, while a "dime" defense adds a sixth defensive back. These extra defenders are known as the nickelback and dimeback, respectively.
The defensive backfield is the area on the football field behind the line of scrimmage. The defensive backs of the secondary are located in the defensive backfield.
The cornerback is a defensive back who protects against the pass and counters wide receivers on the offense. Cornerbacks must follow receivers that run routes on the field to prevent a pass from being caught.
The free safety is a defensive back that stands on the weak side of the field opposite the strong safety. This player is usually the last line of defense. A free safety will protect against the pass.
The difference between a strong safety and free safety is:
The strong side of the field is the side that the tight end lines up on. It's used for play design in football. The strong safety stands on the strong side of the field.