What’s a custom domain name?
A domain name is an address that people use on the internet, with extentions such as .com .net .org (example sergiovargott.com) whether for websites or for e-mail. It’s a string of characters which usually spells out a word, a company name or person’s name. You can buy domain names from us.
How it works:
Tumblr allows it’s users to have their own custom subdomain to run their blog on, the subsomain is the first part of the URL after the http:// (we’ll use sergiovargott.tumblr.com as an example) so (sergiovargott.) is the subdomain. The domain name is tumblr.com which follows the subdomain in the URL and controls it as well.
If you already own a domain name you can use it instead of the subdomain for your blog, so instead of it being sergiovargott.tumblr.com it could be sergiovargott.com.
If you would like to set this up you need to register a domain name, and follow the directions that tumblr gives you very carefully to set it up. See instructions here.