Requests concerning software with Domain Name System (DNS) features as specific requirement should use this. Please check the full tag wiki first if unsure.
DNS (Domain Name System) is a service for the Internet, comparable with a phone book: while in the latter, you lookup people to get their phone numbers (and maybe living addresses), a DNS server can be queried for host names to get their IP addresses.
If DNS is a specific requirement for your request, you should use this tag. Avoid it however if it's rather a natural feature "as a side effect" (e.g. a web browser of course must be able making DNS requests).
When unsure what this is all about, read more on the Wikipedia page on DNS.