A more thorough explanation:

A lie detector test is a device that measures the involuntary physiological changes of a person's body as they respond to questions or statements. The test is based on the theory that when a person lies, their body will show different physiological responses compared to when they tell the truth. The most popular lie detector test used in the United States is the polygraph, which measures factors such as heart rate, blood pressure, respiration, perspiration, and skin conductivity.

The administrator of the test first differentiates between the subject’s physiological response to answering a question honestly and answering dishonestly, and then looks for those responses as a series of questions are asked. Different examiners utilize different questioning techniques. Usually, the subject of the test will be asked direct questions and control questions and their response will be observed as they answer. Alternatively, the subject’s response may be observed as they hear and react to statements or information that are read out loud to them.

The use of lie detector tests is controversial because their validity can be questionable. The test is incapable of differentiating between the physiological factors that often accompany dishonesty and those factors that may simply be a result of the subject’s anxiety at undergoing questioning. Moreover, countermeasures exist that may be employed by subjects to help them ‘beat’ the test.

Although the results are usually inadmissible as evidence, lie detector tests can nonetheless be useful in interrogations during criminal investigations. Many confessions have been made after a criminal suspect fails a lie detector test. However, any results of a lie detector test conducted on a criminal defendant that become publicized can be very influential on public opinion of the case, especially if picked up by the news media.

For example, a police officer may use a lie detector test during an investigation to determine if a suspect is telling the truth about their involvement in a crime. However, the results of the test may not be admissible in court as evidence.

Another example is an employer who is prohibited by law from requiring employees or prospective employees to submit to a lie detector test. If an employee decides to submit to a test, the employer cannot take adverse employment action against them based on the results. However, some states allow exceptions for embezzlement concerns, in which case the testing procedure is heavily regulated and often monitored.