Rexx stands for Restructured Extended Executor.

History

It was created by IBM's Mike Cowlishaw.

What It Can Do

Rexx is an unusual language, often termed as a "meta-language" since its primary function is to control other applications which feature a Rexx interface, allowing complex batch jobs to be completed.
For example, this could mean that a database package could interface with a 2D graphics package to draw a graph of your data. The language is common on older systems where there are packages with limited features.