CNAME stands for 'canonical name'

CNAME stands for 'canonical name' and is a redirection to the zone file of the entered target domain. All entries (Nameserver, MX record, A record) therefore apply to this target domain.  A CNAME record lets you assign a domain or sub-domain to another address or hostname.

For example, testing123.com is your domain name. If you are worried about someone typing your domain with www, like www.testing123.com usually that would result in a page not found error. In order to combat this, you could set up a CNAME record on your domain for www.testing123.com to point to testing123.com so that even if someone typed www or without, they would still be redirected to the correct location (testing123.com) by the CNAME record.