DNS stands for Domain Name System. This system is in place to organize and identify domains. DNS is the phone book of the Web. While a phone book translates a name like “Acme Pizza” into the right phone number to call, the DNS translates a URL or web address (such as “www.google.com”) into the correct IP address to contact (for example “18.104.22.168”) in order to get the information that you want (in this case, the Google homepage).
For some Google Apps functionality, you need to adjust DNS records on your domain. For example, DNS is used to determine where to deliver email for your domain with MX records. To start using Gmail with your Google Apps domain, you need to point your MX records to our mail servers first.