Overcome gross distraction and strong dullness
You can stay focused on the breath more or less continuously, but attention still shifts rapidly back and forth between the breath and various distractions. Whenever a distraction becomes the primary focus of your attention, it pushes the meditation object into the background. This is called gross distraction. But when the mind grows calm, there tends to be another problem, strong dullness. To deal with both of these challenges, you develop continuous introspective awareness to alert you to their presence.