MQTT is as sexy as it sounds. It's an open OASIS and ISO standard lightweight, publish-subscribe network protocol that transports messages between devices. The protocol usually runs over TCP/IP - however, any network protocol that provides ordered, bi-directional connections can support MQTT.
If you're doing anything with the Internet of Things, you can't get away from dealing with MQTT. This Maker Monday Basics video with our amazing Shaun will help you do just that. It's just a little over 4 Minutes long.