This is an example of a library. You may see a list of other libraries used on Rosetta Code at Category:Solutions by Library.
Wren-upc is a module which adds support for splitting a string into user-perceived characters (or grapheme clusters as they are more formally known) to the Wren programming language. It consists of 2 classes: Properties and Graphemes. Only the last one should need to be imported directly by the user.
It is the eleventh in a series of modules (listed on the language's main page) designed to assist with writing Rosetta Code tasks so the same code does not have to be written or copy/pasted time and time again thereby bloating a task's script code unnecessarily.
To use it all you need to do is to copy the source code (linked to in the talk page) to a text file called upc.wren and place this in the same directory as the importing script so the command line interpreter can find it.