Unspam makes it harder for spammers to automatedly send spam to your blog by changing the names of the fields in the comment forms.
What this means for spammers is that they have to do quite a lot more work to send spam to your website. It may also make sending spam to your website unreliable as changes to your theme may upset their spam submission tools. Or they may have to resort to using humans to send spam to your website (not much I can do about that I'm afraid) which will cost them more money.
This plugin is still in beta and so I'd appreciate your help in testing it.