Its basically a modification for apache which filters malicious code, stopping those kids from uploading web shells onto your box.
Mod_security allows you to monitor the attack attempts and in some cases/setups allows you to automatically ban the attacking IP.
Becareful with mod_security as it can mess with the average day PHP scripts if the configuration is a little aggressive. ( Logs help a lot in this area )
I can log up to 4-5 attacks a day via mod_sec, its a great mod for apache
mod_dosevasive is also a good script to install just for peace of mind.
Good luck with the install