Credits
Almost any credit is probably better than no credit, but when an agents asks for credits they usually want you to know whether you've ever been paid for writing fiction.
A good short story credit or two, meaning short stories in magazines the agent has heard of, or that are important in the field, tells the agent you can write fiction well enough to beat out some stiff competition, and well enough to make an editor part with some money.
Winning, or placing high, in a respectable contest does the same thing.
It tells the agent you know how to write fiction, you have a grasp of grammar, punctuation, story structure, characterization, dialogue, etc.
It means the agent is taking less of a chance in requestion a partial or full manuscript.
In the end, the novel you write must speak for itself, but having credits can mean a much betetr chance that the agent will actually ask to see the novel, and that it won't be read first by a junior bottle washer who may reject it before the agent ever sees it.
It is easier to get your novel in teh hands of a top agent or editor if you have some solid credits, but writers with no credits manage to sell novels each and every year. Good credits merely make it easier to get read by a top agent or editor. Good credits won't sell a bad novel, and lack of credits won't stop a good novel.