This is a plugin for SpamAssassin that attempts to detect mail bots that send spam and malware. It does so by looking at headers and testing DNS lookups for classic signs of bots.
You need to do some simple set up which is documented in the .cf-file itself. There's also readme files included in the package.
As with any single SA rule you shouldn't trust it too much as in setting a really high score for it. I recommend testing it with 0.1 and when you have it correctly set up you can raise that to 1.