Is there some place on the web I can go to understand annotations? I really have no idea what they're about. I look in my existing code and see "@GET" and "@Produces" and "@Path" and "@Context" and "@PathParam", and I just scratch my head and think, "Huh?" Is there some place I can go where the idea gets explained for beginners?
"Start with..." is right; that tutorial is pretty sparse. It doesn't really answer the questions "Why would I want to make my own annotations anyway?" and "Okay, suppose I made an annotation, how would I work with it?"
I have provided a detailed treatment on annotations including custom annotations, annotating an annotation, and inheriting annotations. All these concepts are explained with appropriate code. It does provide answers to questions like why create your own annotations and many more. I am not sure whether I am allowed to reproduce some text from the book without a prior permission from the publisher.
ps: As pointed out by Rob, the annotations that you are looking for are not part of Java SE and are thus not covered in the book.