The root zone holds information on the various gTLDs (Generic Top Level Domains) and sTLD’s (Sponsored Top Level Domains).
A fantastic explanation by ICANN, illustrates:
The DNS translates domain names that humans can remember into the numbers used by computers to look up its destination (a little like a phone book is used to look-up a phone number). It does this in stages. The first place it ‘looks’ is the top level of the directory service – or “root zone”. So to use www.google.com as an example, your computer ‘asks’ the root zone directory (or top level) where to find information on “.com”. After it gets a response it then asks the “.com” directory service identified by the root where to find information on .google.com (the second level), and finally asking the google.com directory service identified by “.com” what the address for www.google.com is (the third level). After that process – which is almost instantaneous – the full address is provided to your computer. Different entitiesi manage each one of these directory services: google.com by Google, “.com” by VeriSign Corporation (other top level domains are managed by other organizations), and the root zone by ICANN. – ICANN