DDoS protection is not something you can achieve very easily – especially not via a WP plugin.
Read up a bit on DDoS and you’ll see what I mean.
Typically the people who control and run the routers and servers which host your website are the ones who would normally be concerned with implementing the hard core DDoS protection – and even then it is difficult to have an airtight solution.
This plugin can do some minor things such as stopping various kind of attacks from certain IP addresses or preventing bots from doing the DoS on your site via XML-RPC but all these are only small measures when it comes to proper protection against “denial of service” attacks.