The NS records specify the domain nameservers used for resolving hostnames to IP addresses. You may ask why do zone files need an NS record at all? Because it basically references itself. This is because NS records may have changed, and other servers need to be updated with the IP/hostname of the new domain nameserver.

Previously, this was pointed at dns1.registrar-servers.com, and was cached at many resolving nameservers. When these resolving nameservers query dns1.registrar-servers.com for the IP of hobnob.social, they see that the NS record has been updated to ns1.digitalocean.com and send the request to DigitalOcean's domain nameservers instead.

We can use a program called dig to get the records from the zone file for our domain:

$ dig NS hobnob.social
hobnob.social. 1799 IN NS ns1.digitalocean.com.
hobnob.social. 1799 IN NS ns2.digitalocean.com.
hobnob.social. 1799 IN NS ns3.digitalocean.com.

The first value is the domain; the second is the time-to-live (TTL) value, which is how long this record should be cached for in seconds. The third value, IN, stands for "internet," and will be present in almost all records. The fourth value, NS, indicates that this record should be treated as an NS record. Lastly, the last portion is the value of the record; in this case, it's the hostname of DigitalOcean's domain nameservers.

There are multiple NS records (and multiple domain nameservers) so that if and when one is down or overloaded, it can use the other domain nameservers.