An Internet address or domain name is the Internet equivalent of a postal address. It is how clients will find your website. A domain name is therefore an essential element to take into account when creating a website. It will usually take this form: www.youdot.io, though sub-domains are possible and will look like blog.youdot.io for example.
An expired domain name is a domain that has been previously registered but for various reasons has not been renewed by its owner after it expired. It then becomes available again for registration.
Expired domain names are especially valuable for SEO since they usually already have backlinks (also called inbound links). It is therefore useful to determine the quality of the domain according to its backlink profile (i.e. external links and information on the nature of those links). You can easily analyze the value of an expired domain name with the metrics available on Youdot (Moz and Majestic).