For example, you want to create a topic like San Francisco. However, you also want to capture every other way people talk about San Francisco such as, "SF", "San Fran", "Bay Area", "49ers" and so on. You would create the topic San Francisco, add some seed terms and click get more. Depending on what kind of phrases are inside your data, suggested terms should pop up (the blue boxes) and you just have to click to accept.
Here is a quick video showing this.