2. Do NOT use DNS services from your registrar or from your web host. Use a third-party DNS provider. By using a third-party DNS provider, you retain the ability to make DNS changes should your registrar fail, and a nimbler recovery should your host fail.
If the registrar fails, your domain name is still registered - at the TLD registry, which is distinct from the registrar. (Lord help us if a registry ever fails...) However, you may not be able to make changes at the registrar, which means you may not be able to change DNS records (if you use their DNS services) or you may not be able to change NS records (if you use DNS services through a third-party or through your web host). Indeed, this seems to be the case for many users in the present situation.
Since you may have to live with your NS records as-is, without being able to change them, for some indeterminate period, you want them pointing to a reliable provider, and preferably one that is NOT your web host. (Same issues regarding disputes - don't put all your eggs in one basket.)
Can someone explain this to me? I don't quite understand. What's a DNS service / DNS service provider?