A relative URL is a web address that specifies the location of a resource in relation to the current page or base path, rather than providing the complete domain and protocol. For example, '/images/photo.jpg' is a relative URL, while 'https://example.com/images/photo.jpg' is an absolute URL. In cybersecurity, understanding relative URLs is important because they can affect how resources are loaded and how vulnerabilities like cross-site scripting (XSS) or open redirects are exploited. Proper handling of relative URLs helps prevent attackers from manipulating resource paths or redirecting users to malicious sites.