A free safety is a player in the secondary that can be found in the defensive backfield on the weak side of the field. The free safety lines up about 12 to 15 yards from the line of scrimmage, and it is considered to be the defense's last resort. A free safety's job will be defined as a play evolves. His initial task is to cover the deep passing routes and keep an eye on the receivers, but if the offense chooses to run with the ball the safety has to defend against the ball carrier, if he gets past the defensive line.
The difference between a strong safety and a free safety is their alignment on the field. The strong safety is on the strong side of the field (hence the name), while the free safety is on the weak side of the field. Strong safeties are usually bigger and stronger, they line up closer to the line of scrimmage and will be more focused on running plays, although as the free safety, he may need to attack pass receivers depending on the play that is being run by the offense.
The safety must be one of the best and most versatile players on the defense. As said before, a free safety's job will depend on how a play develops. Because of that, safeties must be excellent play readers and quick decision makers. Quarterbacks will often call plays that confuse safeties, so it is important to know the quarterback's movements and plays. A good safety also has the combination of speed, athleticism, and tackling ability.