Middle pic is a battery alarm -- plug the battery's balance cable with the metal holes up into the left side of the pin header. It should beep loudly (cover the beepers at the top with your finger as you plug it in) report the number of cells, the total voltage then the voltage of each cell.
On the back of the board, between the speakers there's a small button to adjust the alarm trip point. press it once to show the current trip point, and press it again to advance to the next option. until you've taken the time to find out how low you can let the battery go and safely return to a good resting voltage, I'd recommend setting it to 3.60. you can likely lower it from that, but that should deplete roughly 80% of the stored energy, giving you a safe margin to land and will keep the batteries happier for longer.
To use it, plug it in and make sure it's secured to the airframe (I've had these fly off and disappear into grass on a crash) then go fly. if it starts beeping when you're in a throttle punch, you're probably still fine, but when it starts beeping at cruise throttle, it's time to land. Bring her in, let the battery rest for 30s or so, and look at the resting voltage of each cell -- a good stopping point is around 3.7v/cell, resting.
The third pic is a alarm-less version of the battery alarm. Plug your balance plug in and it reports the total voltage and the voltage of each cell.
