In GPO a link refers to a "reverse property". In other words, when object A has a reference to Object B, Object B is said to hold a reverse "link" to Object A.
Since more than one object may be referencing some other object, Link Sets are defined by the property "key" used to reference them.
So, if two objects, A & B reference object C using the property key "parent", then the Link Set of C for the property "parent" will contain objects A & B.
Keep thinking about that until it starts to make sense.
You should see after a while, that the combination of properties and Link Sets provide the basis for a One To Many association structure. And also, as it happens, for the basis of automatic dependency management.
The logical connection is implemented directly in the referencing structures allowing very efficient navigation of the linked structures, for both set processing and dependencies.
These are implemented using an intermediary object - similar in function to a join table in a relational database.
In GPO however, these intermediary objects are dynamically saved within one of the two joined objects, so there is a minimal processing overhead.
The LinkSet object may be retrieved and used directly to order its members if required.