RCEnvironment is a preference pane that allows a user to edit their ~/.MacOSX/environment.plist file. This file is simply a property list of keys and values that the login system will read and load into the process environment of all applications that are launched when the user logs in. These variables are the same as environment variables that can be created in a command line shell (eg: sh or csh), but they also can be seen by GUI applications. In this, these environment variables are somewhat similar to Windows' Environment User Variables.
What's new in RCEnvironment
Version 1.4:
Built with a 64-bit Intel binary to work with the Snow Leopard® System Preferences.
This has been obsolete for years; the environment.plist file this is a GUI for no longer exists or is supported in Mac OS X at all, not for several versions how.