1. Two ways IoT devices interface with the internet that aren't specific to IoT are HTTP/S and WebSockets.
2. MQTT stands for Message Queueing Telemetry Transport.
3. An MQTT client is capable of connecting to a broker, subscribing to topics, process incoming messages to topics they are subscribed to, and publishing to topics.
4. An MQTT broker is capable of letting MQTT clients connect, making sure published messages go to all clients that are subscribed to the message's topic, and publishing messages to any topic.
5. The Pi gets a copy of the messages we send to AdafruitIO because it is subscribed to the topic it is publishing to, and every device subscribed to the topic is sent the message by the broker.