If we’re being picky, then, “to summarize” in a general, non-academic context usually means to simplify.
To summarize in this sense is to touch on all the most important and interesting pieces, to highlight them or to communicate them to someone who is unable to read the original text. In this kind of summary, you’re usually looking for coverage – you want to hit all the main points, and usually in the order you found them in the original. You sacrifice depth for breadth, and that often means leaving out the complicated parts.
Students tend to have come to college with more or less this notion of what a “summary” should look like, probably because they’re used to textbook writing. In textbooks, by definition, very complex ideas are simplified, because the purpose of a textbook is to convey large amounts of general knowledge, rather than to further our knowledge in specific, new directions. So a textbook summary tends to focus on coverage of all relevant main ideas and may leave out many complexities or nuances, so that you get a complete overview, rather than depth on any particular point. Students may sometimes be asked to do this kind of summary for a very simple assignment, when the goal is only to show that you read the text, for example.
But it’s usually not what the professor is really looking for.
The reason summarizing gets tricky at the college level is in the academic context, where our main goal is to think critically about what we know and don’t know and why–not just memorize facts–the most important and interesting bits of a text are not simple, and shouldn’t be simplified, as that would deprive them of their interest and importance. Usually, in academic writing, we summarize another work in order to question or elaborate on its conclusions in a new context. If we start with a simplified version of our sources, our own analysis can only be superficial, and very likely inaccurate!
So, when you’re attempting to “summarize” a text that you will use as a source in your own paper, you need to do something much more complicated than just hitting all the main points in their original order. You want to engage with the text in depth, not just skim its surface. This is why in my own classes I use the more precise term “to distill,” which is a metaphor for exactly the action we want in an essay – a taking out of selected bits, without changing their nature.*
When you distill a source that you want to use in your own essay, you usually do not need to cover every key point of the text. Since the source text probably wasn’t written on purpose to be used as a source in your essay, and in fact had different goals of its own, parts of the source text may not be relevant to your essay. Those don’t need to be covered, then. Instead, you want to hone in on the parts of the source text that directly relate to your goals for your essay. And when you explain these relevant ideas, you want to very deliberately avoid simplifying them. Focus your energy on explaining what is complex, interesting, controversial, incomplete, or questionable about the source text, because it is these nuances that you will want to develop in your essay. This is what we mean by “analysis,” another potentially confusing word you see a lot in assignments—when you analyze a text, you apply your own thinking about the source texts, evaluating their assumptions and sources and goals and logic. You can’t do that if you’ve ignored all the details from the source text.
This confusion about what we mean by “summarizing a source” in an academic essay is actually not a minor matter of semantics at all. When a student summarizes source texts in the sense of simplifying them, the student leaves him- or herself with ideas that are too small and too simple to work with. So the student has nothing to add, and therefore no argument. And next thing I know, I have a stack of essays to grade that were supposed to be analytical, but a huge percentage of them have no argument at all. That is a sad state of affairs for us all!
* I got the term “distill” and countless other useful ways to talk about writing from the University Writing Program at Columbia University, directed by Joseph Bizup, who trained teaching fellows like me. It’s a great term that has served me well in the years since.