The legal phrase double jeopardy stems from the United States Constitution/Amendment Five to the United States Constitution: "...nor shall any person be subject for the same offence to be twice put in jeopardy of life or limb;..." This was meant to limit prosecutorial abuse by the government in bringing repeated actions for the same offense as a means of harassment, oppression or retaliation.
This law is occasionally referred to as a legal technicality, especially when it is used as a criminal defense. It is not uncommon, for example, for police to uncover new evidence proving the guilt of someone previously acquitted of a serious crime. There is little they can do in this case, because the first acquittal is final and the defendant may not be tried again despite the new evidence.
Some conditions that might be seen as double jeopardy are not considered so by the courts. For example, a mistrial does not constitute jeopardy, and separate offenses (perhaps in separate jurisdictions) arising from the same act may be prosecuted separately. For example, a state might try a defendant for murder, after which the federal government might try the same defendant for a federal crime (perhaps a civil rights violation) related to the same act. In another famous case, a man who bribed a judge was tried again for murder after having been acquitted. The appeals court ruled that his first trial didn't really put him in jeopardy because of his bribe.