To novelists, the back story is everything that happened before Page 1. It's Bilbo's life before Gandalf carved that burglar's rune on his door. It's Luke Skywalker repairing droids on his uncle's farm. Though it may contain significant events, the back story mostly lies outside the scope of the story at hand.
But sometimes the back story is interesting in its own right, to the reader or the novelist or both. The Lord of The Rings has among the most complete back stories ever written, including as it does the languages, epic poetry, and mytho-historical background of several different races and their creator deities. The Star Wars back story evolved into three additional movies.
That's what this section is about. From time to time something I'm writing might be enriched by additional background that for whatever reason doesn't seem to belong in the main blog space. You'll find annotations and other expansions here.