The top-level domain is the last part of a URL (Uniform Resource Locator, commonly known as a web address) which identifies either the type of url (eg .com, .org) or a country (eg .uk, .ca).
The types of URL include:
.AERO - air transport industry
.BIZ - business
.COM - Commercial or Company
.EDU - US Educational establishment; other countries use .EDU.<country_tld>
.EU - European Union
.GOV - US Government; other countries use .GOV.<country_tld>
.INFO - Information
.NET - Network
.ORG - Non-profit organisation; also under .ORG.<country_tld>
With country top level domains, they are two letter codes which are intended to suggest that the URL relates to some organisation within that country. However with some countries, the two letter code are "impressive" enough to be wanted to be used by those outside the countries as a type; one in particular is that for Tuvalu or .TV which would be something a TV streaming service would be very interested in using when based anywhere in the world.
The .US tld for the United States does exist, but is not very common; fr example where countries use .CO.<country_tld> to indicate a local company, US companies ([almost] exclusively) use .COM even though it can be used by non-US companies.