In its hayday, WorPress Firewall 2 was an excellent plugin for thwarting the efforts of hackers. What it did was investigated website requests with WordPress-specific heuristics, in order to identify attacks and prevent them from happening. Unfortunately, the developers have decided to discontinue support for the plugin and it is no longer a good option.
An excellent alternative to this plugin is the Simple Security Firewall plugin. This plugin is chalk full of security goodness. Perhaps the best way for you to get to know what it does, is to just watch this video.