What does “First point of canon divergence” mean?

Most fanfic by nature changes canon events. “First point of canon divergence” refers to the time at which a story first changes canon. An alternate seventh book first changes canon in seventh year – up to that point, it uses the canon story exactly. A retelling of the entire series changes canon either before Harry goes to Hogwarts or during the time covered by the first book (SS/PS). A post-DH story that disregards the epilogue changes canon events after Harry leaves Hogwarts, which is referred to as “DH – epilogue” on the site. The only stories that do not change canon events are missing moments and post-Hogwarts stories which conform to JKR’s epilogue.