If something is in-universe, it exists within the Marvel Multiverse. Characters, locations, and items that are officially licensed by Marvel are examples of in-universe things.
If something is out-of-universe, it exists in the real world and is not owned my Marvel. Writers, illustrators, self-made creations, and specific issues of comics (ie Transformers and GI Joe) are examples of out-of-universe things.
In-universe articles should be written from the perspective of a resident of the Marvel Universe. There should be no mention that any of the topics are fictional, except perhaps in a separate section at the bottom of the article. Wikipedia articles are a great source of information for the MDP, but they are written from an out-of-universe perspective and should be thoroughly edited before becoming a part of this project. Instead of saying "Spider-Man was a fictional character from New York", you would say "Spider-Man was from New York", for example.