It's not "cannon" as in a long-range artillery weapon, but "canon", as in an officially recognized literary work.
Such as Biblical Canon is the series of books that are officially recognized as being a part of the Bible. Or American Literary Canon is what is officially recognized as classic American literature.
Therefore, comic canon is what has officially been printed in a Marvel Comic in its most basic definition. However, in most usages of canon, it means what has officially happened in mainstream Marvel continuity, meaning Ultimate books are not considered canon, nor are alternate realities such as the What Ifs or 1602.