thanks, so, because the LED has almost no resistance, and the voltage stays the same, the amperage gets very high, but our microcontroller can not provide more than ~20mA (without overheating it) so we us a resitor. I understand that, thank you!!
My calculations make it a 250 Ohm resistor, but a 220 Ohm should work as well??
(Now, we want to keep it below 20mA (=0.02A) at 5 volts, so we use Ohms law: R = V / I = 5 / 0.02 = 250)