This may be a dumb question, but I think most new users of the site would find it helpful. What is exactly intended as "Word of God" statements?
They're statements from the author of a work establishing some particular part of canon in their fictional universe. The statements are outside of the fiction in question, but they still count as canonical because the author said so.
J. K. Rowling likes to do this a lot by making statements about Harry Potter on Twitter, for example.
The first place I heard the term was TV Tropes, and I believe our usage is the same as theirs:
A statement regarding some ambiguous or undefined aspect of a work, the Word of God comes from someone considered to be the ultimate authority, such as the creator, director, or producer.