Error templates are the messages used to explain to a user why they cannot do something specific on the site, there are a number of different types of error templates. For Example
Access Denied : This is shown when a user does not have access rights/needs to login to see content.
404: The error message when a piece of information is no longer available (usually due to an outside link to your portal).
Changing the text of an error template:
In order to change the text in an error template go to the overview page and click on the name of the template you wish to edit. Change the text as necessary and save.