HF is a WIP API fuzzing tool.
Prof. Regehr has the best 'quick intro' to API fuzzing:
https://blog.regehr.org/archives/1269
There is a dearth of tools available for API fuzzing of real-world software today. Haskell's QuickCheck is good:
https://wiki.haskell.org/Introduction_to_QuickCheck1
but few developers live in that world. Furthermore, QuickCheck is oriented towards checking a simple procedure: if there are properties to verify on a data structure built up over many API calls, it is less suited to the task.
HF allows one to write an "abstract" test program along with a set of 'Generators'. The Generators produce one of any number of values and are intended to sample the space of possible inputs to your API. The Generators are user-driven, however, so you can create only those inputs that make sense for your specific API.
(For those familiar with QuickCheck, 'Generators' are rather similar to 'Property' functions.)