In 'Pulp Fiction', Ringo is one of the two small - time criminals in the diner. He and his partner Yolanda (also known as Honey Bunny) hold up the diner where Pumpkin (Ringo) shows his inexperience and naivete in the criminal world. Their presence in the diner creates a tense situation that gets intertwined with the other storylines in the movie's non - linear narrative.
I'm not sure who 'Ringo' specifically refers to in relation to 'Pulp Fiction'. It could be a name of a character that has some sort of hidden or less - known connection to the movie. Maybe it's a nickname for a character in a different version or interpretation of the story.