A CNAME record is a DNS record which points a name to another name, instead of directly to an IP address. The name pointed to will then have an A record, which points to an address. So if you change your IP address in your A record, all your names with CNAME records will begin resolving to the new IP address without having to be individually changed.
The alternative solution is to have multiple A records, but then you would have multiple places to change the IP address which increases the chances of error. The most common names used in CNAME records are www and ftp.
