In a JVM, each object needs to keep track of some meta info, such as its class, its GC state and if it is used as a monitor in a lock operation. This information is referenced so frequently that it makes little sense to store it anywhere other than in the header of the object itself, accessible by dereferencing the object pointer. Typically, an object header contains lock state, garbage collection state, and type information.

See also Lock word and Class block.