In the Franciscan sense, penance is the grace of effort we receive from the Lord to turn ourselves to God and away from our selfishness and self-involvement. This is how Francis described his own conversion at the beginning of his Testament, and how he described the life of penance in his Letters to the Brothers and Sisters of Penance, or his Letters to All the Faithful, depending on the edition you have.
One of the funny things about penance is that the penance we are given is not usually the penance we imagined ourselves doing. What we are given to do in order to turn ourselves to God and away from ourselves turns out to be something we don't really want.
This is because the penance we picture ourselves doing when we imagine ourselves as fine disciples of the Lord often turns out to be just one more form of selfishness, and in the more dangerous form of spiritual pride. Penance is hardly penance if it feeds our sense of self-satisfaction and makes the flesh feel like it's accomplishing something spiritual.
The devil is perfectly happy to have us do great things for the Lord, provided we can be made to do them in order to feed our own pride and lusts for recognition and approval. But because we are serving ourselves and not the glory of God, we will be constant grouches for lack of receiving the miserable rewards we seek. Our discipleship we will be opaque, and even little children will be able to tell that we are fakers.