From the README.md on GitHub:

> (aka "Algebraic JavaScript Specification")

It's a JS library to implement algebras in code. And:

> An algebra is a set of values, a set of operators that it is closed under and some laws it must obey.

> Each Fantasy Land algebra is a separate specification. An algebra may have dependencies on other algebras which must be implemented.

So that is it. It's "weird", it has a lot of functional concepts (which may or may not be present already in the language -- I'm not a JS-guy myself). It sure must be interesting working on code that does all the things this project does, and I can see it must've required it's share of effort.