When you summarize, you provide your readers with a condensed version of an author's key points. A summary can be as short as a few sentences or much longer, depending on the complexity of the text and the level of detail you wish to provide to your readers. You will need to summarize a source in your paper when you are going to refer to that source and you want your readers to understand the source's argument, main ideas, or plot (if the source is a novel or play) before you lay out your own argument about it, analysis of it, or response to it.