ALIAS-records are virtual alias records resolved by Simple DNS Plus at at the time of each request - providing "flattened" (no CNAME-record chain) synthesized records with data from a hidden source name.
This can be used for different purposes - including solving the classic problem with CNAME-records at the domain apex (for the zone name / for "the naked domain").
To create a new ALIAS-record, right-click a zone in the left list of DNS Records window, and select "New ALIAS-record" from the pop-up menu.
For more details on this record type see https://simpledns.plus/kb/2