This is a pranayama practice that is used to clear the mind and bring clarity. It may feel a bit odd to start but try it out a few times and feel how it effects your peace of mind.
As with most breathwork, with regular practice, alternate-nostril breathing can bring better balance to your nervous system and lower stress levels. Deep breathing and alternate-nostril breathing slow your heartbeat and lower your blood pressure. It may also improve the equilibrium between left side and right side brain.
How to use this technique:
Place your thumb over your right nostril
Exhale fully and slowly through your left nostril
Release your right nostril and put your ring finger on the left nostril
Breathe in deeply and slowly from the right side
Here's a great video from Adriene Mishler that shows the technique