What is Slave Mode and How It Is Used in Lighting and Photography?

When talking about photography, specifically lighting, Slave mode is mode that instructs a flash unit to monitor incoming light, and fire when it senses the light produced by another flash unit firing. Slave mode can be used to fire an off camera flash using the cameras built in flash unit, or to trigger a flash unit as part of a group when you don’t have enough flash receivers. The biggest limitation of slave mode is that it requires direct line of sight between the slave unit and another flash, this can restrict it’s use in certain situations.