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.