The green light, under normal conditions, emits a steady light when server power is on. The amber light emits, under normal conditions, a "heartbeat" blinking light when the server is operating normally. The left-most light is also amber and represents the Local Area Network (LAN) signal. Under normal conditions, this light blinks irregularly as dictated by LAN signal activity.
Making yourself familiar with each of the lights and how they perform under normal and abnormal situations is very important. Take the advice above and then go and check your servers yourself and see how they are functioning.
Oh, and a piece of advice. If you need to show someone your datacenter and you have a dell or HP server with and amber error light, hit the ID button that is intended to make it so you can easily find the server from the other side of the cabinet that you are working in. It will change the flashing amber and make it a flashing blue. That trick has caught on at work and now if you want in, half of the servers that one of the other teams runs has flashing blue lights now.
Great advice. Its really confusing for me to remember the significance of various lights and still i get confused but its really mandatory to get familiar with lights if you are working in a DC.