An ounce is a unit of measurement referring to 28 grams of marijuana. It is believed that there is about 0.7 grams in a typical cigarette, leading to about 40 joints per ounce of cannabis. However given that most joints vary in size there is no way to say this officially.
An ounce has historic meaning because typically cannabis has been purchased in recreational amounts in one ounce or fractional ounces. So a “quarter” is a quarter of an ounce. An “eighth” is an eighth of an ounce.