The purpose of summary is to take many words and condense them down to a few (rather than, say, paraphrase, where your paraphrase probably has about the same number of words as the original—perhaps even more!). In addition, you want to try to use your own words rather than just quote lines from the text verbatim.
First, as other educators have noted, you should read the entire chapter so as to get a sense of everything it tries to convey or accomplish. As you read, it's a good idea to pay close attention to any banners or subheadings, as well as anything in bold print or italics, because these are good indicators that the textbook writers are trying to draw your attention to those terms, topics, and quotations. You may want to highlight or take brief notes on anything you think seems especially important.
Then, read back through this same chapter and look for the topic sentence of each paragraph. It will be the one that seems to provide the main claim or the broadest idea (like an umbrella, where all the other sentences seem to support it). Often, it's the first or the last sentence of the paragraph, but not always. Once you have located most of the topic sentences, you can begin to group them together into even bigger-picture ideas. For example, if there are five paragraphs addressing the five most important elements of Enlightenment thought, each paragraph will likely have a topic sentence that focuses on one element. You can probably combine these five elements into one of your own sentences. Try to combine as many of these topic sentence ideas as you can to keep your summary brief. Once you are finished with this process, go back and check those visual cues (bold type, italics, colors, and so on), as well as any notes you took about what seemed really important earlier. See if you think you've hit everything, or if references to any of these are missing and need to be added.