It's added to the director's room, per guest, by default. It visualizes the packet loss of a guest:
green == good; red == bad; and there is a gradient of colors in between
Other modes support the signal-meter, but you need to add &signalmeter to the URL to show it. You can also do &signalmeter=false to disable it in the director's room.
The signal meter turns into a flame when the remote computer is being throttled by the CPU, instead of the network. This represents the remote computer is overheating or overloaded. Only supported by guests using Chrome really.