Glue records are A records that are associated with NS records to provide “bootstrapping” information to the NS records nameserver.
See RFC 1912 section 2.3.
domain.com. IN NS ns1.domain.com. domain.com. IN NS ns2.domain.com. ns1 IN A 11.33.55.77 ns2 IN A 22.44.66.88
NOTE: In the above example we are mapping each NS records to IP address (A record) thus binding nameservers to IP (that is glue them).