blake, here is what Apple says "The SMU (System Management Unit) is a microcontroller chip on the logic board that controls all power functions for your computer. If your computer is experiencing any power issue, resetting the SMU may resolve it. The SMU controls several functions, including:
Telling the computer when to turn on, turn off, sleep, wake, idle, and so on.
Handling system resets from various commands.
Controlling the fans.
Note that resetting the SMU does not reset the PRAM. Resetting the SMU will not resolve issues in which your computer is unresponsive—in these situations, restarting your computer will generally suffice. If your computer isn't responding, perform these steps one at a time, in this order, until the issue has been resolved:
Force Quit (Option-Command-Escape).
Force Shut Down (press the power button for 10 seconds).
Resetting the SMU can resolve some computer issues such as not starting up, not displaying video, sleep issues, fan noise issues, and so on. If your computer still exhibits these types of issues even after you've restarted the computer, try resetting the SMU. To reset the SMU on one of these iMacs:
Turn off the computer by choosing Shut Down from the Apple menu, or by holding the power button until the computer turns off.
Unplug all cables from the computer, including the power cord.
Wait 10 seconds.
Plug in the power cord while simultaneously pressing and holding the power button on the back of the computer.
Let go of the power button.
Press the power button once more to start up your iMac.
The original iMac G5 model has a small button on the main logic board that will also reset the SMU, allowing you to reset the SMU easily if you have the back cover removed. The original iMac G5 will start up when pressing this button. From here You can find out more about what the LED's mean by reading through this site
Image attached is from an original iMac G5, just to show where the switch is located for that model. Hope this helps, good luck.