During Passover only unleavened bread (matzah) is allowed, but foods containing barley, corn, wheat, spelt and rye are strictly prohibited during the celebration. These grains are used to make leavened bread, which is prohibited during Passover.
Leavened bread is made when flour is mixed with water and is allowed to sit for 18 minutes or longer. After this time, the dough begins to rise and this results in leavened bread. Unleavened bread, on the other hand, is not given a chance to rise; instead, flour from the grains above are mixed with water and baked before the 18-minute mark. This way the dough is not given a chance to rise. This unleavened bread is called matzah.
During Passover there are a variety of foods that can be consumed, including fresh fruit and vegetables, eggs, fresh beef, poultry and fish. Sugar and dairy are also allowed, but these items must be new packages that were not opened before the beginning of Passover.
Avoid foods that were possibly produced with commonly used grains, especially wheat and corn. Read food labels for any mention of corn, such as high fructose corn syrup, corn oil, etc. Corn, in its various forms, is used in processed foods. So, during Passover, avoid junk foods like ice cream, which contains high fructose corn syrup, or potato chips, which are fried in corn oil. Stay away from leavened bread products like muffins, bagels, and rolls.