Relative Strength is the strength per unit body of mass produced by a given individual under specific conditions. In other words, it's about being Bodyweight Strong.
Bodyweight-based exercises are used to enhance relative strength. From simple Squats, Lunges, Push Ups and Pull Ups, to more advanced exercise such as performing a Muscle Up, An Iron Cross, Pistol Squats, and various Crawling and Tumbling Patterns. Many bodyweight exercises can be loaded with external mass which challenges the body to perform the same bodyweight-based exercise (loaded squat, loaded lunge, loaded pull ups etc) but under more force, which then means the body must develop a greater relative strength to move more than its own mass.