The image shows a Description for Function dialog box, which has the following text:

Returns the value of inputString after translating every character
to its lower-case correspondent.
Usage: xp20:lower-case(inputString as string)
Example: xp20:lower-case('ABcId') returns 'abc!d'
For more information please refer to this page. 

The word page is the link to function's description at the World Wide Web Consortium at www.w3.org.