jq is a command-line JSON processor.
If you want to learn to use jq, read the documentation at http://stedolan.github.io/jq. This documentation is generated from the docs/ folder of this repository.
If you want to hack on jq, feel free, but be warned that its internals are not well-documented at the moment. Bring a hard hat and a shovel. Also, read the wiki: https://github.com/stedolan/jq/wiki
To build jq, run
./configure make sudo make install (optionally)
If you've just checked out the latest version from git (rather than using a released source tarball) then you'll need to run this first: