Here's a list of the functions of the pins on the LTC3108 Breakout Board. 
Pin Name Function
V-IN Input Voltage, range is 20 mV to 400 mV.
GND Board Ground.
C-STORE EXTERNAL Auxiliary External Storage Capacitor, positive side.
Discharge Provides discharge path for storage capacitors if needed, e.g. taking board out of service.
VOUT Output Voltage, set to 3.3V by design of the board.
VOUT2 Switched output 3.3V for sensors, etc that don’t have a sleep mode, controlled by VOUT2_EN (provides for MCU control of VOUT2).
VLDO 2.2 V power for MCU in sleep mode.
PGD Power GooD is high (2.2V) when VOUT is within 7.5% of 3.3 V, goes low (0V) when VOUT drops 9% below 3.3 V.
VOUT2_EN VOUT2 enabled when high, 5M internal pulldown resistor.
Here is how the pins on the board relate to the block diagram of the LTC3108 (click to enlarge).