DNS is like a phone directory that resolves a domain hostname (instead of a person's name) to an ip address (instead of a phone number).
Your Cpanel DNS is the server that host your zone information. Unless you have configured your DNS server to be located elsewhere, the Cpanel's DNS section is where you will access to configure your site's DNS.
Most of the entries are pre-populated for you so unless you need to point your mail server elsewhere, there is no need to change the basic entries like www, mail and MX.
Here are some entries in a generic zone. (123.123.123.123 should be your hosting server's IP provided to you)
www A 123.123.123.123
mail A 123.123.123.123
@ MX mail.yourdomain.com
