Domain Extension also calls as Top Level Domains or TLDs are the suffixes of the last part of a domain name.
Whenever you use a domain name in a Url and email, or wherever, it ends with the domain name. This is the last part of the name.
It helps to make the difference creating the best web presence and getting lost in cyberspace.
It also helps with your small business credibility.
Example of extension...
.com is the domain extension or TLDs
SECOND LEVEL DOMAIN EXTENSION
With prefix to TLDs
Eg: co.in , net.in , org.in