A resistor is a basic electronic component that resists the flow of electric current. It is used to control how much current passes through part of a circuit.
Why resistors are used
Resistors can protect components, set signal levels, divide voltage, control brightness, create timing behavior, and make circuits more predictable. A simple example is using a resistor to limit current through an LED.
How resistance is measured
Resistance is measured in ohms. A higher resistance value allows less current to flow for the same voltage. That connection is closely related to what an ohm means.
In plain terms, a resistor is the circuit part that says, "slow the current down to a useful amount."