A relay module is a set of components that is electrically operated and works based on a signal. That can be connected to an Arduino or a transistor and or any other application that the output is a signal or a voltage. Same as relay the relay module is used to control high voltage electronic devices. A Relay module is a mechanical switch which is electrically operated by an electromagnet. When the electromagnet is activated with a low voltage, that can be 5 v, 12 v, 32 v, … it will triggers a mechanical arm that pulls a contact to make a connection between two contacts. Relay modules are used for high voltage controls, and big loads. Relay modules has low power lose in a circuit. In other hands they are slow and they are not quick as transistors.
Arduino is an open-source hardware and software company, project and user community that designs and manufactures single-board microcontrollers and microcontroller kits for building digital devices.