Fasting is going for a certain length of time without eating anything. Sometimes fasting is required before you have surgery, like getting your wisdom teeth removed.
Often fasting is done for medical or health reasons, or in an attempt to lose weight. For many other people, fasting is an important part of their religious observance. Fasting is done by Muslims, Christians, Buddhists, and Jews, among other groups. Fasting comes from fast, which in turn has an Old English root, fæsten, "voluntary abstinence from food or drink, especially as a religious duty."