Resistors
This page will tell you everything you need to know about resistors.
A resistor is an electrical component used to create resistance in the flow of an electrical current. The amount of resistance a resistor has is measured in ohms. Resistors are important as supply too much voltage to a component you can completely destroy it. Resistors can be used to prevent this with just a little bit of math.
To make calculations easier, I have linked 2 tools to help you calculate exactly what you need and what you have.