This is a special in-house type of DNS record specific to the Route53 service.
For an AWS user an Alias record is a great alternative to a CNAME (for supported resources).
Some of the main advantages are:
Aliases resolve directly to an IP address, saving the extra lookup which a CNAME would require
Alias records are supported at the zone apex, so you could create an alias which uses the top of a domain (for example
Alias records allow advanced Route53 features such as weighted/latency/geo routing and failovers
There is no AWS cost associated with Alias lookups
NB: A Route53 Alias record can currently only point to a limited set of AWS resources. For more information please see: http://docs.aws.amazon.com/Route53/latest/DeveloperGuide/resource-record-sets-choosing-alias-non-alias.html