Perhaps this is not the correct place to add this, but because I stumbled on this myself, it may help others to understand:
New Catholic Encyclopedia (2003): “Mortification. The deliberate restraint that one places on natural impulses in order to make them increasingly subject to sanctification through obedience to reason illumined by faith. Jesus Christ required such renunciation of anyone who wished to come after Him (Lk 9.29). And so mortification, or what St. Paul calls the crucifixion of the flesh with its vices and concupiscences (Gal. 5.24), has become a distinguishing mark of those who are Christ’s.