What it does is disallow those who spam to register on your forum or even post, it does this via 3 methods, IP, email & username. You can set options as to how to identify spammers, anyway It's a very good free tool created for phpBB, MyBB, vBulletin, SMF (all popular forum platforms).
It helped me to identify someone was trying to gain access to a restricted area on the blog to spam, I searched his IP and BAM it came up with the stopforumspam page straight away.
So, I suggest if you have very big boards or just don't want loads of junk members, you give it a go.