Some MicroPython boards support the ability to act as an I2C 'slave' or peripheral. This means they can be assigned an I2C address and listen for requests from other I2C devices. You could use this for example if you're creating a sensor in MicroPython that exposes its data to other boards with I2C.
Currently the pyboard is the only board that supports I2C slave mode. In particular the ESP8266 MicroPython port does not currently support I2C slave mode. Check out the pyboard documentation for more details on I2C slave mode usage.