A more thorough explanation:

Definition: A lie detector, also known as a polygraph, is a device that measures and records involuntary physiological changes in the human body during interrogation to evaluate truthfulness.

For example, when a person is hooked up to a lie detector, the machine will measure changes in their heart rate, blood pressure, and breathing patterns. If the person is lying, these physiological changes will be detected and recorded by the machine.

Although polygraph results are not admissible as evidence in most states, they are commonly used by the police as an investigative tool to help determine whether a suspect is telling the truth or not.