What is glue?
Glue is an address record supplied by a parent zone alongside a delegation, to break a circular dependency.
If example.com is served by ns1.example.com, then finding example.com requires finding ns1.example.com - which is itself inside example.com. The parent solves this by including ns1.example.com's IP address as "glue" in the referral. Nameservers that live outside the domain they serve generally do not need glue.