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-rat is a module which adds rational number support to the Wren programming language. It consists of 2 classes: Rat and Rats. The latter contains static methods for dealing with lists of Rat objects.
It is the eighth 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 you need to copy the source code (in the [talk page]) to a text file called rat.wren and place this in the same directory as the importing script so the command line interpreter can find it.
As there is a dependency on the Wren-trait module, you also need to copy that (if it is not already present) to the same directory as described [here]. Unless you are using classes in that module directly, there is no need to import them into your script and the Comparable class can even be imported via Wren-rat itself.