What are glue records?
If the nameserver (NS) is a subdomain of the domain you want to configure, e.g. you want to set ns1.yourdomain.com and ns2.yourdomain.com as nameserver for yourdomain.com, then your domain will need “Glue records”.
The glue record is an A-record to your nameserver’s IP, stored in the parent NS,
mostly the NS for the top level domain (.com.au, .com,.net …)
For a more complete explanation have a look on Wikipedia