A 15 mm LED has two legs, the long one is the Positive and the short one is the Negative.
It consumes 15 milli Ampere of current and can glow on 3 Volts DC Current, or a 3 volt battery.
LED’s are Diodes and the current can only pass through the POSITIVE side.
Always remember to maintain the POLARITY of the LED, that is (plus to plus) and (minus to minus) otherwise it will not work.
LED is a one way street. Now you know how to connect an LED in your circuit.