BPMs are breathes per minute, the rate of breathing. In the breathe and relax exercise, you can set your BPM to 3, 4, 5 or 6 BPM depending on your comfort level. You can of course go at your own pace if you find 6 BPM strenuous too.
There is a lot of evidence connecting breathing rate to stress levels. When we get anxious or stressed, our breathing rate increases, but fortunately it works both ways and we can consciously use the breath to reduce our stress levels.