Summarizing is when you take a longer passage, from several sentences to several paragraphs or even more, and restate the essential main ideas in your own words. When you summarize, you end up with considerably less words than the original source.
The concept of writing a summary is not usually difficult--however, the problem is that many people do not realize that they need to include a citation when they take several paragraphs from a source and turn it into two or three sentences. If the summary is an accurate overview of what the original source said, then it contains the original source's ideas--therefore a citation is required.
A common use of summarizing is when you want to mention an article that you read for your research without getting into too much detail. Here is another example courtesy of the Walden University Writing Center:
In their research, DeBruin-Parecki and Slutzky’s [sic] (2016) studied current U.S. pre-K standards, which are meant to set up students for success in kindergarten and beyond. The authors collected quantitative and qualitative data from diverse survey respondents about pre-K learning standards. The key finding from this study was the positive viewpoint most pre-K teachers have of the national learning standards.
Note that the author names and the year are mentioned in the first sentence, the next sentence starts with "the authors," and the third sentence includes "this study" to reinforce that all three sentences are referring to the same original source.
Citation rules for Summarizing: