"In-universe" is an adjective that refers to the perspective from which the elements of a fictional story are viewed. An in-universe perspective means that how these elements are viewed is limited to the context from within the fictional story. Compare and contrast this concept with out-of-universe.
Most information relating to in-story elements (characters, teams, items, etc.) that appear in the Marvel Database must be written using an in-universe perspective. For instance, the History section of Spider-Man's article will treat him as if he was a real vigilante living in New York City, and will not make references to the fact that he was created by Stan Lee and Steve Ditko, nor make direct references to the comics in which his adventures are published.