The secondary in football is a group of defensive backs on defense:
The secondary is positioned in the defensive backfield or at the line of scrimmage at the start of every down.
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.
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 linebacker is a versatile defensive back that can protect against the pass, blitz the quarterback, or tackle the ball carrier depending on the play.
The strong safety is a defensive back that stands on the strong side of the field opposite the tight end. A strong safety will protect against the pass.
The free safety is a defensive back that stands on the weak side of the field opposite the strong safety. A free safety will protect against the pass.
Strong Safety vs Free Safety
The difference between a strong safety and free safety is:
Football Strong Side
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.