To make use of Group Policy, you don't really have to understand how it works under the hood. You configure GPOs, which contain settings, and then you instruct Active Directory on who or what those GPOs need to apply to. Then, when those computers and users are connected to the corporate network, and therefore connected to Active Directory, they will automatically receive those GPO settings and put them into place on the computers. In other words, Group Policy processes those settings automatically.

What is very important to understand about Group Policy processing is the hierarchy that it follows. As with most Microsoft technologies, Group Policy processing follows a tree-scheme, where the application of settings flow down branches of a tree. There are four levels—also known as tiers or branches—in which Group Policy processing happens.