According to the U.S. National Archives, to "plead the Fifth" means to decline to give self-incriminating evidence. "The Fifth" makes reference to the Fifth Amendment within the Bill of Rights of the U.S. Constitution, which gives a person the right to refuse to answer questions that implicate him in a crime. This right applies both during interrogation and during appearances in a criminal trial.
The relevant clause in the Bill of Rights declares that no person "shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law." Legal experts note that this amendment arose out of a need to protect prisoners from torture as well as those whom law enforcement officials detain. The Fifth Amendment also ties into Miranda rights, which notify a detained person of his right to remain silent when being held by law enforcement officials. The Fifth Amendment and Miranda rights matter because anything that a detained person says is potential evidence to be used against him in a court of law. However, a person who considers pleading the Fifth in a court case must know that the law considers this right to silence waived if he decides to take the stand in his own defense. Witnesses who do plead the Fifth are not to be considered guilty in court unless other evidence presented successfully implicates them.