When you choose a duration, your screen will gradually go dark. If you turned on white noise, it will then play. After the duration has passed, a bell will sound and the screen will return back to normal.
If you are new to all this, Kelly McGonigal has written an excellent post on the basics of mindfulness.
Research suggests that mindfulness practices are useful in the treatment of pain, stress, anxiety, depressive relapse, disordered eating, and addiction. Mindfulness has been investigated for its potential benefit for individuals who do not experience these disorders, as well, with positive results. Mindfulness practice improves the immune system and alters activation symmetries in the prefrontal cortex, a change previously associated with an increase in positive effect and a faster recovery from a negative experience.