ground - Star point in schematics

I read somewhere that in a mixed signal ckt , the analog part and the digital part are supposed to be better when they have different power rails and ground lines , but to avoid floating of the ground , the different ground lines and power lines have to be connected at a point called the star point , so that there is a single ground point and power point. But can someone explain this in a better way , I basically want to understand why a star point is used and its advantages in a mixed signal board .