It can be identified with life force.
When you are healthy, your biofield is hamonized.
When you feel upset, disturbed or angry there is a disturbance in your energy field
This is how accupuncturists describe a sickness.
If you check the EFT techniques, you know as well that tapping on specific pressure points is aimed at clearing these disturbances in your energy field.
This tells you that thoughts and emotional states are totally linked with your biofield state.
Someone who is extremely healthy and vital has of course as well a very harmonized biofield.
All that makes total sense, right?
http://vitalcoaching.com/biofield.htm