This is a Canonical Question about DNS glue records.
What exactly (but briefly) is a DNS glue record? Why are they needed and how do they work?
Answer
A glue record is a term for a record that's served by a DNS server that's not authoritative for the zone, to avoid a condition of impossible dependencies for a DNS zone.
Say I own a DNS zone for example.com. I want to have DNS servers that're hosting the authoritative zone for this domain so that I can actually use it - adding records for the root of the domain, www, mail, etc. So, I put the name servers in the registration to delegate to them - those are always names, so we'll put in ns1.example.com and ns2.example.com.
There's the trick. The TLD's servers will delegate to the DNS servers in the whois record - but they're within example.com. They try to find ns1.example.com, ask the .com servers, and get referred back to... ns1.example.com.
What glue records do is to allow the TLD's servers to send extra information in their response to the query for the example.com zone - to send the IP address that's configured for the name servers, too. It's not authoritative, but it's a pointer to the authoritative servers, allowing for the loop to be resolved.
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