In the context of fiction, the canon of a fictional universe comprises those novels, stories, films, etc. that are considered to be genuine (or "official"), and those events, characters, settings, etc. that are considered to have inarguable existence within the fictional universe. Usually items that are considered canon come from the original source of the fictional universe while non-canon material comes from adaptations or unofficial items. In layman's terms, one could basically say that something that is canon is something that "actually happened" in that universe.
Most, but not all, comic books published by Marvel Comics are set in a shared world known as the Marvel Universe. The canon for this world comprises all the comics not stated to be set in an alternate universe, except those specifically contradicted by later stories. The events may not have occurred exactly as shown, however, owing to the floating timeline (For instance, during the 1960s, Ben Grimm said he had fought in the World War II alongisde Nick Fury; during the 2000s, Grimm himself considered that the idea of him fighting in the World War II was ridiculous, as he would be much older).
Alternate universes in Marvel Comics include, for example, the "Ultimate" line of Marvel comics, which have their own canon independent of the core Marvel universe.
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