In American football, SS stands for "strong safety". The strong safety is usually the larger and stronger of the two safeties, and provides extra protection against run plays by standing closer to the line of scrimmage, usually on the strong (tight end) side of the field. The strong safety is also responsible for helping the corners with deep-pass coverage.
In soccer, SS stands for "supporting striker". The supporting striker participates in attack as well as drops deep to help in defense. Most of the time, the supporting striker passes the ball to those who are in better positions to score.
In football, SS stands for "second striker". The second striker plays slightly behind the center forward, and his main job is to support the attack by dropping deeper. The center forward, on the other hand, is usually the most advanced player in the team and is responsible for scoring the goals for the team.
