A source annotation in an annotated bibliography fulfills a similar purpose to an abstract: they’re both intended to summarize the approach and key points of a source.
However, an annotation may also evaluate the source, discussing the validity and effectiveness of its arguments. Even if your annotation is purely descriptive, you may have a different perspective on the source from the author and highlight different key points.
You should never just copy text from the abstract for your annotation, as doing so constitutes plagiarism.