Help screen language overrides Description
From Joomla! Documentation
Overrides allow you to change a core Joomla string.
In the Joomla code, text strings that are to appear in the User Interface, either the Site interface or the Administrator interface, are expressed as string constants. For example, the string Your session has expired. Please log in again. is expressed as JLIB_ENVIRONMENT_SESSION_EXPIRED. The text string can be translated into any language. The default language is British English. There are thousands of such strings in a Joomla installation.
If a string does not suit your site you can use the Language Override feature to replace the original.