The Group Policy client running on a computer is responsible for processing policy and requesting updates to policy. To ensure that policy changes you make are applied as expected, you need a strong understanding of:
How inheritance works and how inheritance can be modified.
How policy is processed and refreshed, and how you can modify these processes.
How policy processing changes when users connect over slow links, and how you can manage slow-link processing.
How you can plan policy changes so that no surprises occur ...