A reaction paper is just what its name suggests: a formal paper explaining your reaction to a peer-reviewed journal article whose topic is relevant to this course. The paper is a review because your reaction will involve judgment or evaluation; it is an analysis because you will focus on the research methodology and results.
Keep in mind that you are writing a formal essay for an academic audience. Like any good essay, your reaction paper should develop one primary idea or perception, support it with specific evidence (examples from your article, etc.), and present both ideas and evidence in clear language and a logical way.