This answer will explain what a CNAME is and what it can be used for.
CNAME stands for 'canonical name' and is a redirection to the zone file of the entered target domain. All entries (NS, MX, A) therefore apply to this target domain.
CNAMES are primarily used to make it easier to maintain domains with the same basic data, or to assign other domains entered in the same zone the same data as the zone itself.