Refers to the 4th revision of the Internet Protocol, where addresses are usually displayed in the form:

The IP address of a device is its unique identifier on a network which allows communication with that device.

A network will expose a few addresses to the outside Internet and machines on that network will communicate via these gateways.

Due to the explosion of devices connected to the Internet there are real fears of IPv4 address exhaustion which has led to the use of addresses.

IPv4 on Wikipedia