When you summarize a source, you give your readers 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).
Here is an example of a well-known passage from the Declaration of Independence by Thomas Jefferson, pg. 300.
We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness. That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed. That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying down its foundation on such principles as organizing its power in such form, as to them shall seem most likely to effect their Safety and Happiness.
In order to summarize this passage, I might first need to look up some words in a dictionary that are unfamiliar, so that I can put the passage into my own words:
"endowed" means given
"unalienable" refers to rights that cannot be taken away
Here's how I might summarize what Thomas Jefferson has said:
In the Declaration of Independence, Thomas Jefferson writes that all people are created equal and have rights given by God that cannot be taken away. These rights include Life, Liberty and the pursuit of happiness. If Government doesn't respect these rights, the people can get rid of the government or create a new one (300).
I could also summarize it as follows:
The writers of the Declaration of Independence stress that all people are created equal and have rights given by God that cannot be taken away. These rights include Life, Liberty and the pursuit of happiness. If Government doesn't respect these rights, the people can demolish the government or create a new one (Jefferson 300).