This addon is a plugin. You need to extract it to your /plugins folder to install it.
Standard Vanilla behaviour allows you to order roles into a certain priority order however it never does anything with this information. This extension aims to remedy this and fix the role behaviour. It will transform the role system into a hierarchical one so that people may no longer change their own role or the role of users of a higher rank (role priority). If their role is given the 'Change Roles' permission then they may edit other users' roles but they may not promote them to a rank on the same level or higher than their own. The role with the highest priority (hopefully the administrator role) is exempt from all of these restrictions.