Breath meditation may refer to the practice of following your breath during meditation.
Many types of meditations focus on following your breath as a way of focusing your mind, and eventually quieting it.
In guided meditations, the guide will instruct you to follow your breath in different ways. They can instruct you to follow the breath as it enters your nose and goes to your lungs, to pay attention to the sensation of your chest or your stomach rising. To pay attention to the temperature of the breath as it enters and exists your nose.
Following and focusing on your breath during meditation, and during other mindfulness activities, is a great way to focus your mind and to allow it to relax.