Herbgrind grew out of an effort to explore the usage of floating point tools like Herbie, Salsa, Rosa, and FPTaylor, on large numerical software. Herbgrind is built on Valgrind, and draws inspiration from FpDebug. It was developed by Alex Sanchez-Stern, Pavel Panchekha, Sorin Lerner, and Zach Tatlock.
Herbgrind is part of the Herbie Project, so it's also documented on the Herbie site. There are some blog posts about Herbgrind that try to explain how it works, at Alex's blog.