[Radar] Add DNS entry to glossary (#19836)

Author: andre-j3sus

src/content/docs/radar/glossary.mdx

@@ -94,6 +94,13 @@ Cloudflare uses a variety of data sources to categorize domains. Using Cloudflar
 
 In some cases, a domain may be miscategorized. For example, a social media site might be categorized as "Shopping & Auctions". If you believe a domain is miscategorized, or a domain has not yet been categorized, please provide your suggested category using [this form](https://radar.cloudflare.com/domains/feedback) to bring it to our attention.
 
+## DNS
+
+The [Domain Name System (DNS)](https://www.cloudflare.com/learning/dns/what-is-dns/) is a network service that is most commonly used to translate human-readable domain names into numerical IP addresses that computers can use to talk to each other.
+It is an essential Internet service, and is also used to look up other network-related information.
+
+The data displayed on Radar for DNS is based on aggregated and anonymized DNS lookups to Cloudflare's [1.1.1.1](https://one.one.one.one/dns/) public resolver service.
+
 ## Domain rankings
 
 Domain Rankings is based on our anonymized and aggregated [1.1.1.1 resolver](https://one.one.one.one/dns/) data, complies with our [privacy policy](https://www.cloudflare.com/en-gb/privacypolicy/), and aims to identify the top most popular domains that reflect how people use the Internet globally. Domain Rankings’ popularity metric is best described as the estimated number of unique users that access a domain over some period of time.