If that file is missing, the server's default behavior is often to generate an automated HTML list of everything inside that folder. This list is what you see as the "Index of" page. The Role of the "Parent Directory" The Power of Google Dorking - Red Sentry
Thus, the phrase is the literal text link you see at the top of these server-generated pages, allowing you to click back up one level to see the broader folder structure. index of parent directory