This is a console cable, also known as a rollover cable. It is used to connect a computer's serial port to the console port of a network device, such as a router or switch, for configuration and management.
It typically has a 9-pin DB9 serial connector on one end and an RJ45 Ethernet connector on the other.
The cable is often used with Cisco devices but is also compatible with other network equipment that has a console port.
It is used to access and configure network devices via a command-line interface (CLI).