These 5 LEDs are voltage indicator lights that can help you diagnose voltage problems with your video card. With the fan facing towards you and the power connectors facing up, the LEDs are, from left to right:

GPU - This indicates if voltage to the video card's GPU is good
MEM - This indicates if voltage to the video card's memory is good
PLL - This indicates if the voltage for auxillary components such as fans, LEDs, etc. is good
3.3V - This indicates if the voltage supplied from the motherboard via PCIe slot is good
12V - This indicates if the voltage from the 12v input from the power supply is good

If any of these LEDs are not lighting up, it indicates a voltage issue with that component or source. Checking your power supply, motherboard, or increasing voltage to the video card may help with any of these problems.