I am in my 20’s and recently broke my upper arm bone during a skiing accident.  They call it a “mal-union” break.  What is this?

A mal-union fracture is one that heals in a less than ideal setting or one that disrupts the joint motion.  Sometimes these are addressed by going back in and re-breaking the fracture or by shaving off the bone that is interfering with function.  In your case, depending specifically where the break is, shoulder joint osteotomies (bone shaving) are not recommended.