By default, due to security reasons, IIS hides detailed information about all errors, but you can enable them when needed.
Open the web.config file for the Website or Application that you want to enable detailed error messages and add the following code:
<httpErrors errorMode="Detailed" />
Now restart the IIS and you will see detailed error messages for your Application.
Default Error Messages in IIS
Detailed Error Messages in IIS
NOTE: You should never enable detailed error messages in IIS for production websites, as this reveals too much information which may lead to different security issues.