After a certain time period (arbitrarily designated by the government under which you live), you can no longer be prosecuted for a crime. For instance, if someone starts looking through archived security tapes and notices that you shoplifted something twenty years ago, the statute of limitations prevents you from being brought to trial.

In the USA, statutes of limitations are set by the individual states. Most high-order felonies like first-degree murder, or kidnapping don't have statutes of limitations, and other crimes like fraud have statutes of limitations of around 3 years.