To reach another person on the Internet you have to type an address into your computer — a name or a number. That address must be unique so computers know where to find each other. ICANN coordinates these unique identifiers across the world. Without that coordination, we wouldn’t have one global Internet.
In more technical terms, ICANN coordinates the IANA functions, which are key technical services critical to the continued operations of the Internet’s underlying address book, the Domain Name System (DNS). The IANA functions include: (1) coordination of the assignment of technical protocol parameters including the management of the address & routing parameter area (ARPA) top-level domain; (2) administration of certain responsibilities associated with Internet DNS root zone management such as generic (gTLD) & country code (ccTLD) Top-Level Domains; (3) allocation of Internet numbering resources; & (4) other services.