It's a git pre-commit script that checks your Javascript code for errors before every commit, if it finds any issues it simply aborts the commit. Sgt-Donowitz helps you set this up in less than a minute.
Because it's easier to maintain good code when you are forced to
from the root of your project run:
curl -s jt.io/lint | sh
Mac OS X / Unix only
If for some reason you must commit a JS file without validating/linting it you can always run a commit with the "no-verify" flag:
i.e. git commit --no-verify .
However, when doing this remember that there are no flags to remove bad Karma.
If you want to learn more about this, I've written a blog post in my blog here