"A domain alias is a domain name that can be used as an alternate name for another domain." - Source: Google
For example, if you have two domains for your flower shop business, you can "map" the second domain as an "Alias" of the primary domain. Website visitors will see the Alias in their URL bar when navigating the site.
Here's how to configure a domain Alias on your hosting cPanel:
Relevance to WordPress
When you have a WordPress website (or website of any kind), you can assign Aliases to your website. However, not all servers are ready "out-of-the-box" to map domains to a single website and require additional configuration.
Furthermore, your WordPress website needs some extra configuration to "map" each alias to a specific "post" in WordPress (or group of posts).
Posts can include any default post type that comes out of the box with WordPress, like Pages, Posts, Products, etc, or any Custom Post Type created by other plugins or themes.
End Goal: Multiple domains on a single WordPress site
At the end of the day, the purpose of creating an Alias domain on your server is to map multiple domains to a single WordPress installation.
Domain Mapping System let's you handle the mapping of domains to specific posts after you've set up your server properly.