Top-level domain (TLD) refers to the last segment of a domain name, or the part that follows immediately after the "dot" symbol.
Some of the TLDs and their explanations are as follows:
.com - Commercial businesses
.org - Organizations (generally charitable)
.net - Network organizations
.gov - U.S. government agencies
.mil - Military
.edu - Educational facilities like universities
.th - Thailand
.ca - Canada
.au - Australia