Alternator Gauge

It's called an ammeter (amp meter) which measures current (in amps) going into or out of the battery. It is normal to see a discharge while idling, especially if you have everything on. When you rev the engine the alternator puts out more current and you see a charge condition on the ammeter. You can add a voltmeter (which measures volts) to get a warm fuzzy feeling that all is well but it won't tell you much.

Your idle may be a bit low and you can bump it up just a bit by turning the idle adjustment and/or the mixture screw on the carb.