Multiple Flash Units
Multiple flash is when you use two or more flash units that are fired in synchronization with the camera's shutter. The flash units can be either auxiliary units that are connected to the camera by extension cords or they can be slave flash units.
Slave flash units generally come with self-contained power sources. They are fired with a photoelectric cell when the light from a master flash unit strikes the cell of a slave unit. When using multiple flash, the exposure calculations are situated on the distance between your subject and the flash unit that gives off the most intense light to the subject. This means you can have several auxiliary flash or slave units and calculate your exposure from the main light source. The rest of the flash units should be at an equal or further distance from the subject compared to the unit the exposure is based on.