The Executive Order establishes the order of succession for the Office of the National Cyber Director. If the current National Cyber Director is unable to perform their duties, the responsibilities will pass down a list of officials, beginning with the Deputy National Cyber Director and continuing through several Assistant Directors and the General Counsel. There are exceptions, such as that individuals serving in an acting capacity or those not eligible under the Federal Vacancies Reform Act cannot assume the role. Furthermore, the President retains the discretion to appoint a different acting Director, and the order does not create any enforceable rights for others.
Simple Explanation
Imagine a team leader has to leave suddenly, so the leader tells everyone in a set order who will take over next. This document is like that plan, saying who will be in charge if the National Cyber Director is not there, but the President can still choose someone different if needed.