Anything central is in the middle of something — or essential to it. Central things are fundamental and important.
Think about the center of a circle: it's right in the middle, equidistant from all sides. Similarly, anything central is in the middle of something. The central part of a city is downtown. You can also say something central is an important, crucial part of something. National security is a central responsibility of the President. Communication is a central goal of writing. Central things are essential and key.