Rollover cable is a type of null-modem cable that is most commonly used to connect a computer terminal to a router's console

System console

The system console, root console or simply console is the text entry and display device for system administration messages, particularly those from the BIOS or boot loader, the kernel, from the init system and from the system logger...

port. This cable is typically flat (and has a light blue color) to help distinguish it from other types of network cabling. It gets the name rollover because the pinouts on one end are reversed from the other, as if the wire had been rolled over and you were viewing it from the other side.

This rollover cable is more commonly known as the Yost cable or more appropriately the "Yost Serial Device Wiring Standard".