The strong safety in football is a defensive secondary player (SS) positioned in the defensive backfield. The strong safety differs from a free safety because strong safeties start on the strong side of the field, which is the same side as the tight end. The strong side of the field has more players.
The strong safety stands on the strong side of the football field on the same side as the opposing team's tight end player. The strong safety is generally bigger and stronger than the free safety.
The strong safety is a member of the secondary and also known as a defensive back.
The secondary is a group of players on defense that stand in the defensive backfield, the area behind the line of scrimmage. The strong safety and his counterpart the free safety serve as a last line of defense if the ball carrier on the opposing team's offense manages to break through the defensive line.
The strong safety is usually larger and more muscular than the weak side safety. However, this is not why he called the strong safety. He is responsible for covering the offensive strong side. He works to stop the run by stepping into the runner's route and blocking, but he also is there to cover the pass. The strong safety has a job of tackling the ball carrier on both passing plays and running plays if they break through the defensive line.
Strong safeties are positioned closer to the line of scrimmage to fight off tight ends and other larger players.
If you want to learn more about the strong safety and the defense in football, check out the following tutorials: