The ShowMissingKeys setting is one of the AppSettings
that is contained in the web.config file.
DotNetNuke has a User Interface Localization
system that allows users to update any localized keys. This is used both within the Core and it's modules, and is a best practice followed by the great majority of 3rd party modules.
This allows users to alter text to values they prefer, either to preferred text or to text in another language. In this way users can even purchase extensions (such as modules) which do not come with localized values for their user(s) and add the relevant translations themselves.
The ShowMissingKeys attribute is used to replace missing localization keys e.g. you have a module that supports both English and German, which has a localization key btnHelp.text that is missing in one of them. When ShowMissingKeys is set to True it will replace the missing text with RESX: as a visual indication to help developers find missing keys.
This setting can be found in the appSettings section of DotNetNuke's web.config file in the root of your website.
<add key="ShowMissingKeys" value="true" />This setting should be set to false for a production site.