When something is repeated, it usually shows the heart of what someone is trying to get across - like when a boss mentions a deadline multiple times in one email or a friend drops the name of their favorite store several times in the week before their birthday. The same thing applies with God's Word. If you see anything mentioned multiple times in the same passage, mark it or make a note - these are your key words. Key words can be anything: a character in a story, a characteristic of God or of people, a group, a phrase, an idea, etc.
There are several ways to mark these words. Marking can vary from person to person and, honestly, it’s all up to you. Some people use symbols (like stars, circles, or rectangles around words) and others use different colors, or a combination of both. Regardless of which method you choose, the goal is to help you slow down and pay attention to what Scripture is saying each time you mark a key word.