A mogul base light bulb is a light bulb that has a larger screw base than a standard screw base bulb. The screw base of a mogul bulb is 39 millimeters across, while the base for a standard bulb is 26 millimeters across.
A bulb that is referred to as an E39 base bulb is one that has a base of 39 millimeters. An E26, or medium base bulb, has a base of 26 millimeters. The "U" in product descriptions means that the bulb is mogul base. There are smaller bases available, such as intermediate, candelabra and mini-candelabra, but the mogul base is the only one that is larger.