NS Record

An NS record says which nameservers answer for a domain. When you set your domain to use Cloudflare, or your host, or your registrar's own DNS, what you're really doing is changing the NS records at the registry to point at those servers.

Everything else, the A record for the website, the MX records for email, the TXT records for verification, lives on whatever nameservers the NS records point to. Change the NS records and all of that comes from somewhere new.

It's the one DNS change that takes the longest to propagate, often up to 48 hours, because the registry's TTL on NS records tends to be long. It's also the change that breaks the most sites, because people switch nameservers before they've recreated their records on the new provider.