Most PCB design tools convert a schematic diagram to a PCB layout. This is actually a fairly complex process. A design tool has to analyze the symbol instances of a schematic diagram as well as how pins are connected by nets. Then it has to associate symbols with footprints. Pins of footprints are, then, connected by a “rat’s nest”.
A “rat’s nest” is a collection of rat lines. A rat line is a logical connection between two pins. While rat lines will overlap without any problems, they cannot be on an actual PCB.