When working in FCPX, that big black bar going across the middle of your timeline is called the p rimary storyline. Its name describes its function perfectly—it's where your primary story should be told. If you're making a documentary, talking heads and essential imagery should be placed here. However, secondary elements (b-roll, cutaways, picture-in-pictures, music, and so on), are usually going to be placed as connected clips above or below the primary story. If you haven't done so already, this would be a good time to read the Creating connected clips recipe in Chapter 3, Basic Editing Mechanics.
Connected clips are great to use and unbound by the rules of the Magnetic Timeline. But sometimes the very properties that make a connected clip what it is, can cause frustrations, especially if you do some heavy trimming. Binding connected clips into a new storyline can help mitigate these problems.