What is a delegation?
Delegation is how responsibility for a domain is handed down the DNS tree. The root delegates .com to the com registry; the com registry delegates example.com to your nameservers, by publishing NS records that name them.
A resolver follows this chain from the top: root, then TLD, then your nameservers. The delegation is the link at each step. If the delegation is missing or points at the wrong nameservers, the domain does not resolve.