In short, specificity refers to the training modalities aimed towards a specific sport or action. This is extremely important when an individual wishes to train for their desired sport or event. When training specific movements, an individual should try to mimic their sport-related movements and actions as much as possible. For example, if a baseball pitcher wishes to increase their throwing velocity, they need to of course strengthen their shoulder and arm, but also train to increase their angular velocity as well. A strong arm does not necessarily allow you to throw a baseball at a high speed, you would need to specify your training to increase that arm/ball speed.
Another aspect that is involved in specificity, and a topic that I have discussed before deals with athletic periodizational training. Athletes generally compete in sport seasons that they can target their training schedule around. For example, after a sport season has completed, an athlete will usually go into a short recovery or "off" time. They will then get back into a training schedule based on how long they have until the next season starts. Early training usually involves a lot of strength and endurance training, but as the training schedule progresses closer and closer to the sport season, this is when the athlete begins to specify their training more and more in order to increase their sport-specific skill set.
Overload refers to the workload being applied to an individual. Some may refer to this as "progressive overload" which essentially means that you need to continue to challenge your body in new ways in sequential training session in order for the body to continue to adapt and improve over time.
Exercise causes micro tears and damage to the body which is then rebuilt and repaired with proper diet and rest in order to become stronger for future use. In order for this to continue to happen, the body must continually increase its workload in order to continue the adaptation process.
Progression is related to overload in the fact that an individual needs to continue to progress with their training by increasing their workload in a number of ways. Workloads can be progressed in a number of ways such as shortening rest periods, increasing resistance loads, increasing workout durations, etc. all based on an individual's specific training schedule.
When combining all three of these training principles, a highly effective training schedule can be created for an individual based on their goals and aspirations to create faster, stronger and sometimes bigger muscles to perform better and more efficiently or fatigue-resistant.