What is a photo slave is and how it is used when taking photographs?

When talking about photography, specifically lighting, a ‘photo slave’ is a flash unit set to slave mode which will fire when it detects the output of another flash unit. a photo slave can be triggered by a cameras built in flash unit, or another flash unit in a group when you don’t have enough flash receivers for the lighting setup you are using. The biggest limitation of photo slaves and slave mode is that they require direct line of sight between the slave unit and another flash, this can restrict it’s use in certain situations.