EPU is an ASUS software for computer power management. It will downclock your cpu and lower the cpu voltage to save energy. It is automatically disabled when you overclock your computer because lowered voltage and increased processor frequency would cause instability. If you want to keep your overclock and you aren't worried about power saving, you can click on the icon and deselect the start up option to prevent the message from showing up.
Thank you so much for this information. After just buying and having this computer built I didn't want to do anything to harm it. From what I could find I knew it had something to do with energy savings but that is about it. Thank you for taking the time to reply to my message.