“…nor shall any person be subject for the same offence to be twice put in jeopardy of life or limb…”
The “double jeopardy” clause is meant to protect individuals from enduring the ordeal of a criminal trial more than once for the same crime. Without this protection, prosecutors who might be ambitious, vindictive, or simply incompetent could use successive trials as a means to advance their own careers, persecute their enemies, or simply harass the public.
In practical terms, the prohibition of double jeopardy means that, if a prosecutor believes he has enough evidence to prove that a suspect is guilty of a crime, then he has one shot at making his case before a jury. If he fails, he will not be given a second chance to make his case, even if he has reason to believe that he could make it better the second time around. Moreover, if he succeeds at winning a conviction, he will not be able to try again later for a more serious charge or a harsher punishment.