Hackers may attempt to send requests to your web site for pages which don't exist. This is because they are blindly attempting to find pages which are known to be potential security hazards.
Enabling intrusion detection will catch efforts to send many requests for non-existent pages. These visitors will automatically be locked out of making further requests for the period of time specified in the options.
We recommend clicking Enable 404 Detection to "On", as well as Enable Email Notifications. The remaining settings can be left at default or changed as you prefer.