This little circuit board is a high efficiency DC/DC step-down converter which can output up to 3 Amp at 5V without the need of any heat-sink or forced cooling. (It does get a bit toasty at 3A though) UBEC stands for "universal battery eliminator circuit" and this UBEC is designed to replace a 5V supply in RC planes and 'copters but its also great for any kind of microcontroller or electronics project that runs off of 5V.
To use, connect 6V up to 16V (recommended) to the IN-side wires and 5V will be generated on the OUT-side wires. You'll get 5V (+-5%) regulated output from higher voltage input. 3A output current, 5A peak - if you're trying to draw 3A you'll need to give it a little more than 6V though, 7V works well. The UBEC has built in filter capacitors on the input and output, but its a switching supply so its a little noisy compared to a linear regulator.