The principle of specificity in training states that to achieve desired results, training must be closely aligned with the specific goals of the athlete or individual. For example, if someone aims to improve strength, their training should focus on weightlifting; if the goal is to enhance endurance, then cardiovascular activities such as running or cycling should be prioritized. This principle underscores the importance of tailoring workouts to meet individual objectives and ensuring that training stimuli produce the desired adaptations in performance or fitness.