NS stands for nameservers and these servers are hosts that are specified at registrar level. These hosts keep record of DNS records of a domain name. These hosts itself holds DNS records that can be queries by fetching DNS records of these hosts. The term nameservers clearly tells that these servers holds names of domains and their corresponding DNS records. Today, many cloud proxies entertains users to point to their nameservers in domain registrar and the cloud proxy service will handle the rest. After setting up nameserver records at registrar, the users need to add DNS records at cloud proxies panel, which can be edited later any time.
The NS DNS tool helps a lot in verifying the DNS records of the nameservers of domain. Just put a domain name and it automatically fetches nameservers and then fetches DNS records of those nameservers to show as a result.