Summarizing the text you've read is an valuable way to check your understanding of the text. When you summarize, you should be able to find and write down the thesis and main points of the text.
Annotating the thesis and main points
Mark the thesis and main points of the piece. The thesis is the main idea or claim of the text, and relates to the author's purpose for writing. Sometimes the thesis is not explicitly stated, but is implied in the text, but you should still be able to paraphrase an overall idea the author is interested in exploring in the text. The thesis can be thought of as a promise the writer makes to the reader that the rest of the essay attempts to fulfill.
The main points are the major subtopics, or sub-ideas the author wants to explore. Main points make up the body of the text, and are often signaled by major divisions in the structure of the text.
Marking the thesis and main points will help you understand the overall idea of the text, and the way the author has chosen to develop her or his thesis through the main points s/he has chosen.