"Story by" involves what?

[Indefatigable]

Out of curiosity, when someone gets a "Story by" credit, what did they actually do? Best I can figure, the ideas for the plot of the episode were thought up by them. But do they actually write any lines?

[Bobbywoodhogan]

I would just think that they are the ones who develop the actual idea of the story, the characters, plot line, etc then someone writes the scrupt.