Glue records are A records that are associated with NS records to provide “bootstrapping” information to the NS records nameserver. (see [[http://www.faqs.org/rfcs/rfc1912.html|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

In the above example we are mapping each NS records to IP address (A record) thus binding nameservers to IP (that is glue them).