"Subproperty of" is a declarative predefined property to define that a property is a subproperty of another and is provided by Semantic MediaWiki. This property is pre-deployed (also known as special property) and comes with additional administrative privileges but can be used just like any other user-defined property.
The special property Subproperty of specifies that the property is a subproperty of another property, much like how a category is a subcategory of another.
See smw:Help:Semantic search 1.0#Subcategories and subproperties.