Debug Console via Windows taskbar icon

Hello!
A question from a newbie:
I'm able to start a Debug Console via Windows taskbar by clicking the Arduino agent icon. Debug Console is shown in Browser with the address http://127.0.0.1:8991/ .

Please tell me, how to use it and what I can do with the send button.

With my running sketch on Arduino it displays again a again the properties of my Arduino Uno.
If I input text and send it with the send button the text is echoed with:
{
"Error": "Could not understand command."
}

Which commands are make sense?

Thank you for advice
Erhy