A slave flash unit is one which is triggered by the light emitted from a primary flash unit. Many such units contain a photoelectric cell which detects the flash from another unit and immediately discharges the slave unit. Others are triggered by an infrared or radio frequency link. It is of course also possible to link flash units using the appropriate cables and adaptors, and use direct wired control. In some circumstances this is convenient and practicable, but in others it may be difficult to conceal the wires and exclude them from the images.
Slave flash units are useful because they allow photographers to add light to any area of an image, and from any direction. Some units are quite small and can be concealed behind objects within a scene typically to provide fill for shadow detail.
There is of course a delay between the firing of the primary flash source and the subsequent discharge of the slave unit. However, this is so short that it is negligible.