In preaching his legendary sermon Edwards wanted to put the fear of God into his audience. He fervently believed that far too many Americans were becoming lax in their moral standards. What they needed was a good old-fashioned dose of hellfire and brimstone to shake them up a bit, to make them change their ways.
Just about everyone at that time believed in the literal existence of Hell; that it was a fiery pit of torment, of endless screaming, wailing, and gnashing of teeth. Most people also liked to believe that they were part of God's elect, those who would be saved from the flames of Hell and ascend to Heaven instead. But they couldn't be sure. So they spent a lot of time worrying about whether they had indeed been saved. Edwards plays upon this fear in his sermon, warning his audience of the terrible fate that awaits them if they should fail to heed his words and repent of their sins.
In immediate terms, Edwards' sermon was devastatingly effective, with many of his audience actually fainting in terror. It also proved its worth in the long-term, helping to facilitate the spread of the national religious revival known as the Great Awakening.