Send a MQTT message directly from Javascript

From this form it is possible to send a MQTT message directly to the MQTT RGB Panel. This for uses the Paho Javascript library and the iot.eclipse.org mqtt server.

MQTT pusher per pannello: LedPanel 1

MQTT LedPanel gets the print commands from MQTT broker from making a subscription to the following topic:

tanzolab/mqtt_panel/macaddress/cmd 

macaddress is mac address of your Arietta G25 WiFi adapter and is unique all over the world so you can use also public MQTT broker like iot.eclipse.org to talk with your LedPanel.

From this topic LedPanel waits for a JSON message in this format:

#Print a string
{
    "cmd"     : "print",
    "font"    : "font filename",
    "color"   : "font color",
    "bgcolor" : "background color",
    "text"    : "text"
}

What is a Topic ? What is JSON ? What is a MAC address ?