Canned response is a predefined answer that can be reused while replying to a case.
The answers will be stored in the form of an Email Template in Vtiger and then when you try to reply to a case you can directly use the stored template from the Canned Response section in the Reply section of a case.
To add a canned response please add a new template for cases module.Menu
> Email Templates
> Add Template
> Select the module for the template as Cases > Save.
To know more about how to add an email template, please check this link