1. What is A Domain?
Domain is the name of your online store or brand with which people remember you with.
example:
www.google.com
www.google.in
www.google.co.in
so this name google you can buy in .com or .in or with many other options that are available .
Now, question is where can you buy these domians?
You can buy these domains from sites hosting company sites like godaddy.com |hostgator.com|hostinger.com|bluehost.com|porkbun.com|namecheap.com and manymore..
so, you reserve your domain for 1year or multiple years as per interest or buy for one year and renew every year.