The on/off switch for the machine is on the left. The small round light on the on/off switch itself just indicates that the machine is switched on.
Immediately to the right of the on/off switch is a rectangular heating element indicator light, which is lit (yellow) when the heating element is on. The heating element light comes on immediately when the machine is turned on, then goes out when the boiler has reached the proper temperature for steaming milk; it will go on again occasionally when the boiler temperature drops, as when you make steam.
On the right of side of the machine is another rectangular indicator light, this one green. It indicates a sufficient water level in the reservoir, so it should remain lit while the machine is on. If it goes off, the heating element and pump will not operate.
On the far right (immediately right of the water level indicator) is the rocker switch that controls brewing. Push it on to start water flowing through the unit, and off to stop water flow. The small round light on the rocker switch indicates that the pump is running.