Biological psychology is the intersection of two different disciplines in our attempt to understand behaviour. The core assumption of biological psychology is that psychology (why we do what we do, think what we think, etc) is biological (our bodies/brains/environments) are all we need to understand if we want to understand psychology). Check out this video to get a taste of what this branch of psychology is all about.
The central nervous system is the operations and control system in the human body. It is fundamental to who we are as humans and part of every experience we have and behaviour we perform. Understanding its basic components helps us understand how our behaviour is always rooted in biological mechanisms: you can't explain any behaviour without talking about the CNS!