Too Long; Didn't Read
Subdomain hijacking refers to a technique by which "unused" subdomains can be made to point to a location of the attacker's choice. This can both affect your site's reputation, and enable phishing attacks leveraging the legitimacy of your domain. It doesn't necessarily validate your DNS records, but some tools exist to help you prevent it from happening. The tool I've enjoyed over the last few years is the "flyover domain flyover" tool created by security researcher Michael Henriksen.