Top-level domains (TLDs) are the highest level of domain names in the hierarchical Domain name system (DNS), such as .com, .org, .love, .guru, .xxx, etc.
There’s a very powerful entity called the Internet Corporation for Assigned Names and Numbers (ICANN) that gets to declare these things legitimate. That means that DNS is not at all decentralized: there is an ultimate authority, with all the political problems that implies.
When a new TLD is approved by ICANN, it is added to the DNS root zone. The root zone is managed by IANA (Internet Assigned Numbers Authority), a department of ICANN. The root zone file is distributed to the DNS root servers, which are operated by different organizations around the world.
ICANN collects fees at various points in this process to pay for the root servers and its other operations.