WeakRefLookupTable is a subclass of LookupTable. It behaves the same as the regular LookupTable class, and has the same methods; the only difference is that the values in a weak-reference table are only "weakly" referenced. (The keys are still "strongly" referenced; only the values are weak references.)
A "weak reference" is a reference that doesn't prevent the garbage collector from removing the referenced object from memory. On each scan of memory, the garbage collector deletes each object it finds that is not referenced at all, and each object that is referenced exclusively through weak references. This means that if an object is referenced only as a value stored in a WeakRefLookupTable, the garbage collector can delete the object. Whenever the garbage collector deletes an object that is stored as a value in a WeakRefLookupTable, the WeakRefLookupTable automatically deletes that key/value pair.
Weak references have their uses, especially in situations where you want to create a secondary access path to a set of objects for performance reasons, such as an index or a cache. These cases are relatively rare, though, so don't feel that you need to go out of your way to find places to use this intrinsic class. If you can't think of a reason to use this class, just ignore it and use the base LookupTable class.