Jack was a bad man. He was extremely selfish. He wasn't helpful at all. He didn't like helping people. He was a miser (= he didn't like spending money).
When Jack died, he didn't go to Heaven because he was such a miser but he didn't go to hell either because the Devil didn't want him there. So Jack was stuck. He had to walk the earth, holding a lantern, until Judgement Day.
This is the story that has been told by the Irish people who came to America in the 1800's. They carved their turnips into the face of "Jack-of-the-lantern" to remind themselves what happened to people who were misers. So that's why Americans today carve their pumpkins into jack-o-lanterns.