In a real fight, you would aim your attacks at the weakest parts of the foe's body, so that ideally one hit could end the fight quickly. Thus the primary targets would be head and neck, while hands, groin, armpits and knees are also valuable.
Without protective equipment - like fencing masks, gauntlets, armor - attacking these targets may be painful and dangerous. However, you and your partner may still like to make some kind of controlled and noninjurious contact to each other with blunt training weapons (e.g. wasters or feders).
Since you might not have all the needed protective equipment available when you just start your training, then in the videos we often target alternate parts of the body which are protected by muscle and fat. Thus we call these body parts substitute targets.
Often, an attack to such an alternate target is more difficult than it would be to an actual target. But this also means: If you can hit the alternate target, then you should be able to hit the actual target.