There are no spoilers in academic writing.
An abstract is a detailed summary of an article/essay/paper, usually 150-250 words. It hits on the premise of the article, whatever methodology might have been undertaken, and finally -- and importantly! -- the conclusion reached in the end. Yes: they will spoil the ending of the article.
When you're doing research, the abstract is the next thing you should read (after the article title, of course), because, obviously, a summary is going to help you decide whether to spend more time on that particular article.
For example, this article's abstract provides  an overview;  an existing argument;  the author's case study;  the author's conclusion.
Remember, abstracts are a preview for the article they're associated with...which means that when you have to write an abstract, you're summarizing your own work (and not your sources).
Save writing it for last. It's easier to summarize what your paper is about after you've written it!