What is Groovy's MetaClass used for?

You're probably thinking of Groovy's MetaClass:

A MetaClass within Groovy defines the behaviour of any given Groovy or Java class. The MetaClass interface defines two parts. The client API, which is defined via the extend MetaObjectProtocol interface and the contract with the Groovy runtime system. In general the compiler and Groovy runtime engine interact with methods on this class whilst MetaClass clients interact with the method defined by the MetaObjectProtocol interface

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The Groovy MetaClass lets you assign behavior and state to Classes at runtime without editing the original source code, it's a layer above the original Class.

It's the mechanism used by Groovy to extend the Java JDK objects.

Example:

Object.class.metaClass.explode{-> println "Boom! ${delegate} Exploded!"}
"SomeString".explode();
12345.explode();

Output:

Boom! SomeString Exploded!
Boom! 12345 Exploded!

For more advanced usage, read this: MetaClasses

from Wikipedia

In object-oriented programming, a metaclass is a class whose instances are classes. Just as an ordinary class defines the behavior of certain objects, a metaclass defines the behavior of certain classes and their instances. Not all object-oriented programming languages support metaclasses. Among those that do, the extent to which metaclasses can override any given aspect of class behavior varies. Each language has its own metaobject protocol, a set of rules that govern how objects, classes, and metaclasses interact.

and

Support in languages and tools

The following are some of the most prominent programming languages that support metaclasses. Common Lisp, via CLOS Groovy Objective-C Python Perl, via the metaclass pragma, as well as Moose Ruby Smalltalk Some less widespread languages that support metaclasses include OpenJava, OpenC++, OpenAda, CorbaScript, ObjVLisp, Object-Z, MODEL-K, XOTcl, and MELDC.