Cats is a lightweight, modular, and extensible library for functional programming in Scala. On its website, it is stated that the name “cats” is a playful shortening of the word, category - so it has nothing to do with the animal: cat (if you ever wondering), but with category theory :)
It is a library that I have personally found to be helpful in making functional programming in Scala less confusing. It does this by being more concrete and more obvious with regards the various patterns and abstractions that arises from a valid logical conclusion of programming with functions in a statically typed language that has higher-kinded types.
Unfortunately, due to the relative unfamiliarity with functional programming and the various abstraction that comes with it, the Cats library is usually approached with hesitation amongst a sizable amount of software developers.
The sentiments I have seen expressed directly or indirectly is that introducing Cats would inadvertently fling developers into the deep ends of various unknown, academic and good for nothing, category theory inspired abstractions, that only confuses and add complexity. Since nobody wants this, the decision usually gets made not to bother with a library like Cats.
Nothing can be farther from the truth.