BEC=Battery Eliminator Circuit. The U in UBEC stands for Ultimate IIRC.
It's just a voltage regulator that turns the main battery voltage into a lower voltage (5v or 6v typically) that your receiver and servos can handle. Without this regulator, you'd have to use a seperate 4 or 5 cell battery pack, which adds weight and something else to charge.
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here for more info, but it's probably more data than you need...