Indicator light on electrical switch
There are switches with indicator lights that illuminate when on and other switches that illuminate when off. The former are called pilot lights, the latter, illuminated switches. 

To determine which yours is, you need to look at the switch without the cover plate and determine which position is ON. While up is the convention for a single pole switch that is completing a circuit, it doesn't really mean anything, since a switch can be mounted upside down.

Most single pole switches have an up indication stamped on the metal mounting frame. If that is positioned up, the switch is connected when the paddle is up.

If there is no indication, you will need a circuit tester to determine which position completes the circuit.