If it's the same Hobbico Box that I just put together myself (And used today) it is showing you the current draw on the battery from your glow plug. Then you also have a knob which allows you to adjust the current flow to the Plug.

If you look at the meter is has a range of colors on it........ when connected to the Glow plug, turn the dial until you are in the green, then fire the engine up.