Pope Francis has called for a day of fasting and prayer for peace in Syria. This call was well received among other religious traditions, some of which will join the Catholics in this particular day of fasting.
Throughout human history, thousands of people have fasted for political, civil, religious and medical reasons. Even Gandhi said that fasting was as old as Adam. Other relevant people who have fasted are Confucius, Aristotle and Martin Luther King Jr.
For most religions, fasting means remorse, openness of the soul and prayer. It's a sign of penitence and union as brothers.
For Catholics, fasting means to cut down the main meal of the day, and reduce the others.