Properties are one of the most fundamental features of an entity; so fundamental that I can probably assume you've worked out what they do.
However, properties can sometimes not just configure the entity they're set on, but also configure other entities.
This is done through property aliases. A property alias essentially forwards any change to a property on one entity to a property of another entity. This can be helpful on its own (for example, a folder ZEntity might be able to configure many different smaller sub-entities in one place), but it also allows for the use of non-root-entity properties by other entity templates.
If you recall that an entity template's root entity's functionality is exposed to any other entity that may incorporate it, then it stands to reason that if the root entity uses an alias to link its properties to another sub-entity, then another entity incorporating it could use that alias even though it connects to a non-root entity. This is the case, as shown in the diagram below: