What Fasting Is:
Fasting, broadly speaking, is the voluntary avoidance of something that is good. When Catholics talk about fasting, we normally mean restricting the food that we eat. We can fast between meals, by not eating snacks, or we can engage in a complete fast by abstaining from all food. The English word breakfast, in fact, means the meal that breaks the fast.
While fasting takes the form of refraining from eating, it is primarily a spiritual discipline designed to tame the body so that we can concentrate on higher things.
Prayer, Fasting, and Almsgiving - The Swiss Army Knife of the Spirit:
That is why fasting is usually mentioned along with prayer and almsgiving (or charity). By controlling the passions of the body, we free our souls for prayer. And by refraining from eating, we free up food or money that we can give to those less fortunate than ourselves. The three spiritual disciplines go hand in hand, and the Church calls us to practice all three together, especially during the season of Lent.
Lenten Fasting and Penance:
Lent, the 40 days before Easter Sunday, is a season of the Church calendar set aside for Christians to do penance in preparation for the celebration of the resurrection of Jesus Christ. Refraining from food can help us to bring our bodies under the control of our souls, but it is also a way of doing penance for past excesses. That is why the Church strongly recommends that Catholics fast during Lent.
Current Church Law Regarding Fasting:
The Church used to prescribe very rigorous rules for the Lenten fast (including abstaining from all meat and eating only one meal per day). The current rules, however, are much more lax. Catholics are only required to fast on Ash Wednesday, the first day of Lent, and on Good Friday, the day that Jesus Christ was crucified. Anyone over the age of 18, but under the age of 60, should eat only one full meal on those days, although they can also have small amounts of food in the morning and the evening.
Going Beyond What's Required:
The Church continues to encourage individual Catholics to observe a stricter fast. Extreme fasting, however, can be physically harmful, so, as with all physical forms of penance and of spiritual discipline, you should consult with your priest before embarking on a very strict fast.