A golden goodie was a confectionary wrapped in golden paper in the form of a gold coin that was produced by a magically-grown bush called the money tree. Each golden goodie had the picture of whoever the bush was sent to inscribed on the wrapper, and produced a rather tantalizing scent that caused whoever smelled it to be attracted to wanting it. It appeared outside Greedy's house in the episode "The Smurfs And The Money Tree", magically planted there by Gargamel's mother in order to test the Smurfs' belief in sharing things with each other, since they have no use for money.
The morning when Greedy awoke to find the golden goodie bush outside his house, he tried one of them and found them to be so delicious, he felt like he had to share them with the rest of the village -- until a small devil Smurf claiming to be "the real him" said it would be better for him to keep them all to himself. However, the smell of the golden goodies attracted other Smurfs, who came to Greedy to ask for one, only for the devil Smurf to tell Greedy (and for him to tell the other Smurfs) that they can't have a golden goodie for nothing. Eventually the other Smurfs decided to give up something valuable of theirs in order to have a golden goodie, which for Harmony meant his horn, Vanity his mirror, Handy his tools, Hefty his weights, and Brainy his glasses. But when Papa Smurf came over to Greedy's house and found out that he couldn't have a golden goodie for free, he turned up his nose and left, disappointed.
Soon the Smurfs who have traded their valuable things for a golden goodie found themselves with nothing left after they had eaten their treat, and so asked Greedy if they could have their things back. Again goaded by the devil Smurf, Greedy told them that they couldn't, saying that a deal is a deal. Realizing that Greedy has really become greedy, the other Smurfs left his house, leaving Greedy without friends but with all their possessions which he tries to use for his own enjoyment.
Outside his house, Greedy saw his fellow Smurfs playing a game with a pine cone, making him see that they weren't so dependent on things in order to have fun, so he decided to give all the things the other Smurfs have given for a golden goodie back to them. This made the devil Smurf so upset that he decided to leave him for good. Papa Smurf saw Greedy's generous side return to him and suspected that Gargamel must have been the one behind the golden goodie bush, so he decided to have the golden goodie bush replanted outside the evil wizard's castle, using his greed and his mother's against themselves.