Rules are a combination of criteria that must be met in order for your menu configuration to appear.
Rules appear in multiple modules within the UI Manager plugin, and you'll find them to the left side of your View configuration.
Rules are cumulative, not exclusive. For example, if you select the administrator role and a specific user, the view will be visible to ALL administrators AND to the selected user, regardless of his/her role. You must select at least one rule, otherwise no one will see this menu view.
Rules also take priority from top to bottom. So, for example, say you have one menu view that has only the administrator role checked. You then create another view that has only the user name "bob" checked. When you log in as an administrator with the user name of "bob, the first menu view will override the second because it appears higher in the rules list.
The rules available to you may change based on the module you're viewing, the type of install you have (single site vs. multisite) or the type of plugins you have installed (like WP Ultimo for example).