How your brain and body functions effect how you act and think. Psychology is how you work mentally, biological psychology is how your physical process effect your mental processes.
It is also known as neuropsychology.
Say your adrenal gland underproduces it's hormone. If it under produces, it can make you depressed, lethargic.
The body effected the mind, causing a depression that might not have been there otherwise.
This particular branch of psychology focuses on neurotransmitions and bodily functions and how these process effect your mental health.