Author notes are small notes from the author that isn't part of the story just to give their followers updates. It usually is in the beginning or end of a chapter, and hopefully never in the middle. They commonly look like this:

(AN: Sorry you guys I won't be updating for a while!)

I really don't think author notes are necessary, since everyone has a message board and you can post updates there instead of interrupting your book, as it may annoy the reader.