Every community website has a temporary website address associated with it, and many communities also have a custom domain.
A temporary address includes .communitysite.com at the end and is less memorable than a custom domain. Websites on a free trial or the free plan will be accessible from their temporary address.
Custom domains are shorter, easier to remember, and useful for creating custom email accounts.
To set up a custom domain for your website, your community must have an active subscription to the deluxe plan. When that condition is met, you'll be able to set one up by following the steps in this article: How to set up a domain.