The Homie convention strives to be a communication definition on top of MQTT between IoT devices and controlling entities.
MQTT is a machine-to-machine (M2M)/"Internet of Things" connectivity protocol. It was designed as an extremely lightweight publish/subscribe messaging transport.
MQTT supports easy and unrestricted message-based communication. However, MQTT doesn't define the structure and content of these messages and their relation. An IoT device publishes data and provides interaction possibilities but a controlling entity will need to be specifically configured to be able to interface with the device.
The Homie convention defines a standardized way of how IoT devices and services announce themselves and their data on the communication channel. The Homie convention is thereby a crucial aspect in the support of automatic discovery, configuration and usage of devices and services over the MQTT protocol.
Find the convention here: https://homieiot.github.io