Annotations are boxes of text and/or links that could be placed anywhere inside a video. Since May 2017 you can no longer add new annotations, and on January 15, 2019, even existing annotations will disappear. This is really unfortunate for older videos that make heavy use of them, providing commentary, background info, or corrections.
If you want to preserve your videos' annotations, you can use this tool to convert them into subtitles instead.
To check which of your videos have annotations, use AnnoFetch.
Almost certainly! I'm just waiting for someone to tell me why.
It's probably not the intended use for subtitles, but it seems better than nothing — at least you can keep the annotation text around in some form.
I wrote all the code myself, but the ideas aren't mine.