Here’s an analogy I use to help explain the concept of Penance in the Catholic Church.
Billy is a 14 year old who likes to ride his bike a lot. So much, in fact, that he even rides in the rain. However, Billy is often absent-minded when he’s riding and forgets the rules set forth by his mother. One of those rules is that Billy must only ride on the side walk and street.
One day, Billy was riding his Bike after a good rain, and he rode his bike off the side walk, onto the grass, and wiped out in his neighbor’s flower bed. Billy was unscathed, but half the flower bed was destroyed, and his tires had made numerous, deep treads in the wet lawn. Billy knew immediately that he had failed to obey, and he could see why his mother had set forth her rules.
So Billy brushed himself off, rang the neighbor’s doorbell, and apologized for his error. The neighbor, Doris, a kind old woman who had known Billy since birth, had no hesitation to forgive him. Then, Billy went home and told his mother what had happened. He apologized to her as well. His mother forgave him and said that she was proud of him for responding how he did. “So,” she said, “because I believe that you are truly sorry, I’m not going to punish you for disobeying.” “However,” Billy’s mom continued, “forgiveness doesn’t change the fact that Doris’ flower bed is mangled. So I want you to go over there tomorrow and help her tidy it up.” Billy graciously replied, “You’re right, Mom; I know that’s the right thing to do.”
Penance is derived from the word penitent. As an adjective, penitent denotes a sense of sorrow for one’s sin and a disposition to amendment; the word contrite is a good synonym. As a noun, penitent refers to a person possessing that sense of sorrow for sin and disposition to change. So it is that penance is an act performed by a penitent person, otherwise simply called “the penitent,” to signify and demonstrate his contrition while also helping him along in his desired conversion.
It should be apparent now that penance is not conceived of as satisfaction for sins committed. Jesus’s death was the solitary satisfaction for sin. His death, however, does not, nor was it intended to, eliminate all consequences of sin, eg. the flower bed is not restored by Jesus’s sacrifice. Rather, penance is the natural consequence of being penitent, of contrition. I think it is beneficial and accurate to think of it as a spiritual discipline undertaken in response to one’s recognition of and sorrow for sin and the desire for spiritual growth. In the Sacrament of Reconciliation, the priest merely prescribes a particular act of penance that seems to him to be in accord with the nature and severity of sins committed.