It doesn't have a specific meaning and is used by Archie as a catchphrase to express various emotions, like:
* Agreement: When someone says something he agrees with.
* Surprise: When something unexpected happens.
* Excitement: When he's looking forward to something.
* Anger: Sometimes he uses it as a way to vent frustration.
The phrase is a kind of verbal tic that reflects Archie's working-class background and his limited vocabulary. It's become iconic and associated with the show and its characters.