DFHack is a Dwarf Fortress memory access library, distributed with scripts and plugins implementing a wide variety of useful functions and tools.
The full documentation is available online here.
If you have DFHack installed, it is also accessible as raw text in the hack/docs folder.
If you're an end-user, modder, or interested in contributing to DFHack -- go read those docs.
If the docs are unclear or you need more help, please check out our support page for ways to contact the DFHack developers.