What is the basis for double jeopardy and what exactly does it do?

just to get it all straightened out, i want to know exactly what would cause DJ (double jeopardy). lets say someone is charged for selling weed. he gets taken to court and somehow is acquitted of the charge. does that mean that in the future he cannot be charged with selling weed? like whatsoever? or is it that for that exact scenario could not be brought back to court as in the same case could not be brought back due to DJ. any other details regarding DJ would be nice as to i am very curious how it works.