As others have already pointed out, a weak reference provides a means for using an object as a key without creating a strong reference to it. This is useful in situations where you don't want to impair the JVM's ability to garbage collect the object, but yet still want the ability to track some aspect of the object, which makes a weak reference ideal for caching or storing metadata about the object.
I'd suggest reading "Understanding Weak References" (Oracle blog article), about strong vs. weak references in Java. Without an understanding of the difference, the data structure itself makes little sense.
