Modules to communicate with other devices. For setting up a wireless connection over WIFI, Bluetooth, 2.4GHz, 433mhz, RFID and Infrared. There are also modules for receiving GPS signals, sending SMS messages or simply converting parallel to serial communication.
Modules are printed circuit boards that often have a specific function, such as measuring the temperature, controlling motors or playing sound. These modules have a connection (usually a header) to communicate with a microcontroller such as the Arduino and Raspberry PI.