To summarize something, only include the important stuff. If your teacher asks you to summarize several events from the Civil War, she wants the important facts, not every single detail you can find.
When you summarize something, you write or tell the general idea and only the most important points. It's a skill we often use in school or at work, but even in our personal lives, we summarize: like when a friend asks what the book you're reading is about, or someone wants to know about your recent vacation. You don't tell them every single detail, talking yourself to exhaustion. Instead, you summarize — describing just the key events that "sum up" the experience.