Quoting Wikipedia:
Jython, successor of JPython, is an implementation of the Python programming language written in Java.
As for what uses it may have:
Jython programs can seamlessly import and use any Java class. Except for some standard modules, Jython programs use Java classes instead of Python modules.
Performance measurements of Jython, IronPython and CPython show that both Jython and IronPython outperform CPython for some cases with large datasets.