A lighting contactor is basically a remote control switching mechanism - a much larger version of a relay. Typically a contactor has switched 'poles' and an electric coil that when energised, closes the poles allowing a current to flow to one or more circuits.
For example, think of football stadium and the floodlighting required to light it. There is no way that you would run a switch for each light. Instead, groups of floodlights are wired through large (really large) contactors. The contactors can then be triggered by a lowly 120V circuit and a toggle switch although in reality a stadium would used something more sophisticated.
Contactors are used in many applications, not just lighting. Your car probably has over 40 relays which are small contactors and large motors used in industry are switched by contactors.
I hope that helps.
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