On supported plans, domain aliases allow you to add additional domains that share the same mail server and email accounts as the primary domain. You will not have the ability to change any settings for the aliased domains because all settings (including email recipients) will mirror the primary domain.
An example use of a domain alias would be when you have multiple domain extensions such as example.COM, example.NET and example.ORG and where all mail server settings for the domains are identical.
Domain aliases are not just limited to domain extensions, but can be used in any case where secondary domains have email configurations that are identical to the primary domain. So, if example.com was the primary domain, but another-example.com also shared the same email server and email users, it could be set up as a free domain alias in SpamHero.
Also, just to be clear, when an email is sent to a user on the aliased domain (e.g. some-user@aliased-domain.com), the recipient of the message will not be changed to use the main domain upon delivery.