Opto-Isolator is a device that uses a short optical transmission path to transfer an electronic signal between elements of a circuit, typically a transmitter and a receiver, while keeping them electrically isolated.
Since the electrical signal is converted to a light beam, transferred, then converted back to an electrical signal, there is no need for electrical connection between the source and destination circuits…
Opto-Isolators are commonly used to isolate field and system power. For example in DI card, field input is first passed through protection circuit and then to optoisolator before transmitting to microcontroller.