MQTT Client lets you connect to a MQTT broker and publish strings to a topic. First create a MQTTClient. You have to pass a task list from Cooperative Multitasking, a client (e.g. WiFiClient from WiFi101), host, port and credentials of the MQTT broker. Then create a MQTTTopic. Use the MQTTClient and the topic name as arguments when doing that. To connect to the broker call MQTTClient::connect(). After that you can call MQTTTopic:publish("...") to publish a string. Requires Cooperative Multitasking for processing the acknowledgement packets of the broker. So you have call CooperativeMultitasking::run() in your loop() function. See the examples for details.