404 detection looks at a user who is hitting a large number of non-existent pages and getting a large number of 404 errors. 404 detection assumes that a user who hits a lot of 404 errors in a short period of time is scanning for something (presumably a vulnerability) and locks them out accordingly. This also gives the added benefit of helping you find hidden problems causing 404 errors on unseen parts of your site as all errors will be logged in the "View Logs" page. After you enable the feature you'll have several options to help tailor configure it.
Minutes to Remember 404 Error (Check Period)
The number of minutes in which 404 errors should be remembered and counted towards lockouts.
The numbers of errors (within the check period time frame) that will trigger a lockout. Set to zero (0) to record 404 errors without locking out users. This can be useful for troubleshooting content or other errors. The default is 20.
404 File/directory White List
Use the whitelist to prevent recording common 404 errors. If you know a common file on your site is missing and you do not want it to count towards a lockout record it here. You must list the full path beginning with the "/".
Ignored File Types
File types here will be recorded as 404s but not lead to a lockout.