Scap loading is scapular retraction. Pulling the shoulder blades in toward the spine.
It's good because the scapula (shoulder blade) is the only kinetic link from the torso to the arm. It can also protect the front of the shoulder against forces that push the arm behind you during shoulder rotation and external rotation.
It's bad if you think you are scap loading by simply get the elbow behind your shoulder.
Scapular retraction is also part of proper form when doing pushups and bench press types of exercises to in order to protect the shoulder.