What is Glue record?
Glue record is a record, that stores IP address assigned to domain or subdomain .It is stored in registrar zone. These records become extremely important when domain uses the names of its namespace as hosts for its nameservers.It can lead to infinite loop.
For example domain.st uses nameservers - ns1.domain.st, ns2.domain.st.
To make it work you need to define glue records for your ns1.domain.st and ns2.domain.st to avoid infinite loop.
The glue record can be found in the "Additional records" section of the DNS response.
To break the loop, you just need the Glue record, which directly will give the IP address of the name servers while processing the request for an address for your domain.
For domains that do not use their own subdomains for nameserver addresses, glue records are also useful, they reduce the number of steps when getting the address