Imagine that you only have 100 Ω resistors at your disposal and that you want to turn on a 2V LED with your 9V battery. If you connect one 100 Ω resistor in series with the LED, you’ll get 70mA through your LED which is way too much your LED (max current for your LED is 20mA). It will become really hot and die!

So how will you turn on your LED?

The solution? Connect four 100 Ω resistors in series, and you’ll get 400 Ω. That gives you a current of 17.5 mA through your LED, so it’s safe.