The answer to 'what is switch' is a switch is a device used to control the flow of electricity in an electrical circuit. It is a mechanical or electronic device that can open or close an electrical circuit, allowing or preventing the flow of electricity.
Switches come in many different shapes, sizes, and types, and are used in a variety of applications, from simple light switches in your home to complex control systems in industrial machinery. They can be manually operated, such as a light switch on the wall, or automatically controlled, such as a thermostat that turns on a heating system when the temperature drops.