Before defining what it should be, we need to learn what it shouldn't first. And, it shouldn't be a summary. You need to realize from the very beginning that writing an APA reaction paper is not only about short description of the piece of literature itself and it's not a brief retelling of its content. Having said that, the following description is about what APA reaction paper should (may) be.
It should be a reflection of things you liked and disliked, of your personal impression, whether it is positive or negative. Feel free to tell the truth about what you think.
APA reaction paper may be in the form of a question, if you are trying to find an answer to what you are interested in; it may be in the form of a constructive criticism, if you find the piece not interesting, not useful or there are significant disadvantages in it; APA reaction paper may also be in the form of alternative suggestion regarding the situation from the piece of given literature.
APA reaction paper should be your practice of capability to form your unique opinion, to agree or disagree with something and provide strong argumentation of your opinion.
APA reaction paper should be your training field for taking stock of the piece of literature, its author, and the life within the piece (the characters, their behavior, situations happening to them, etc.) and for looking for additional or alternative solutions regarding issues raised in the piece.