This is a LED. It stands for Light Emitting Diode. The long leg is the positive (anode) side and the short leg is the negative (cathode) side.
Another way to identify is that the cathode has a flat surface on the LED.
I choose to bend my anode like a lightning bolt, and my cathode like a 0 (zero).
One thing I Iearned today is that different colors of LEDs use different amounts of energy. Therefore it may be necessary to double stack batteries, run a circuit in parallel, and/or choose a lower power LED color.