LEDs require a minimum voltage to work. Different LEDs require different voltages. Most indicator LEDs use between 2 to 3-1/2 volts. The current to drive these LEDs must be limited to keep them from burning out, usually 5 to 30 milli-amps.
Example; 3 volt battery, 2.5 volt 10ma LED
voltage drop(E) current(I) resistor to limit current(R)
Ohm's Law R=E/I
R=.5 / .010
R= 50 ohms
P=(I)squared * R
P=.0001 * 50
P=5 milliwatt
A 1/8 to 1/2 watt resistor should work fine.