A domain name has at least two parts, the top-level domain (.com, .org, .edu, .design), and the second-level (amazon, google, craigslist). In the example Amazon.com, “Amazon” is the second-level and .com is the top level.
What the heck is a third level then?
Well, when you add another qualifier to the left of the second-level, that is the third level. For example, many website builder sites like Weebly offer free third level names. So I might get Andrew.weebly.com just to try out their service or because I want to build a site affordably.
Porkbun, in conjunction with Weebly, also offers free third level domains via its various .design sites. We operate graphic.design, web.design, interior.design, fashion.design, architecture.design, and industrial.design.
So, while I could purchase amazing second-level names, like Andrew.design, I might also consider andrew.graphic.design or andrew.web.design, etc., which are all available on those respective sites. Whoever owns the second-level (graphic.design) is able to control all levels to the left (andrew.graphic.design), which explains why we are able to offer third level names. These free third-level names have less functionality, cannot support email, and are inherently longer, but they are a good way to test out a site and are especially appealing as a stepping stone for folks as they start a new project.