The so-called GC (Garbage Collector) roots are objects special for garbage collector. Garbage collector collects those objects that are not GC roots and are not accessible by references from GC roots.
There are several kinds of GC roots. One object can belong to more than one kind of root. The root kinds are:
java.lang.Classhappen to be roots of other kind(s).
If an object is a root, it is specially marked in all views showing individual objects. For example, the following picture shows a fragment of paths view: