I guess if there’s any one thing you figure you “know how to do” already, it would be breathing. You’ve been doing it successfully for a while now, yes?
But we adults have often gotten away from breathing the way a child or an animal does: using our diaphragm and the lower part of our torso (“belly”) to initiate inhales and exhales. Why? Tight pants, self-consciousness, spending a lot of time sitting…there are multiple reasons.
Belly breathing noticeably helps during endurance workouts (consciously try it the next time you hit that tough hill on foot or your bike). It also is great during any “mindful breathing” activity like meditation, or straight-up breathing exercises aimed at stress reduction or sleep inducement.