Lambdas in Ruby (or procs if you will, the difference is not important in the present context) are powerful abstractions that make it possible to write functional Ruby code in general and higher-order functions in particular.
The present example demonstrates a module written using nothing but lambdas, where its entry point itself is a lambda. Note how the implementation does not define any methods and the actual lambdas doing the calculations are stored in variables accessible only to the entry-point lambda, in effect creating a closure.
The application in turn uses the imported object as a callable.
This example also demonstrates the power of export_default, which can be used
to hide a ton of complexity behind any Ruby object.