On the forum you will find many explanations of Erel on this.
But quickly I can tell you that it is a very simple and fast protocol that has been designed for iot devices, that is, the transmission has a low consumption of Bytes and a great reliability. Undeliverable messages remain on hold and there are three levels of security for message delivery.
Unlike the WebSocket which is 'one to many', MQTT works as 'many to many', similar to a chat, every message sent by a client is automatically forwarded to all those listening on the channel.
Then it is clear that with the appropriate code the WebSocket you can make it work as many to many and also MQTT you can make it work as one to many.