It's called a Cisco Console Cable or Cisco Serial Console Cable.
One end is RJ45 - looks like normal LAN plug, but it's not for internet. That end goes into the "CONSOLE" port on Cisco device. Every Cisco switch/router has this blue port labeled CONSOLE.
The other end is DB9 Serial - the old 9-pin connector with screws. That end goes into COM port of laptop/ PC. Newer laptops don't have COM ports anymore, so you add a USB-to-Serial adapter to make it USB.