The aim of isolators in electrical engineering is to protect end users and expensive electronic systems and components from high voltages by galvanic isolation.
It is to ensure that subsystems with large differences in ground potential can communicate effectively and sensitive signals are protected from corruption by electrical interference (noise).
Isolators allow a control or data signal to be coupled across an isolation barrier without current flowing through it.
Signals are transmitted, but interference is blocked.

An optocoupler / a Solid State Relay (SSR) is an electronic device which is used to transmit a signal between two isolated circuits.
Optocouplers / SSRs consist of an optical transmitter, typically an infrared light emitting diode (IR-LED), and an optical receiver, for example a phototransistor / MOSFET / triac, which are both separated in a housing through a transparent insulating material.
Depending on the type you are used for digital or analog signal transmission.

Isolation amplifiers have one or more isolated signal path(s) for amplification or conversion of analogue current and voltage signals and, when equipped with their own power supply, also supply paths for the input and output circuits, at least one of which is also isolated.

Digital isolators are integrated devices used to isolate digital signals and transfer digital communication across an isolation barrier.