An integration or channel provides the user interface for your chatbot.
For example, out of the box, Dialogflow ES supports a lot of channels such as Facebook Messenger, Slack etc.
And if you want to create an “integration” between Dialogflow ES and any channel which is not supported by Dialogflow ES out of the box (e.g. Zoho SalesIQ), you can use the Custom Payload to send the rich responses supported by the specific channel you are using.
Creating a custom integration actually requires that you write some code. While the out-of-the-box integrations provided by Dialogflow ES run on Google’s servers, in the case of custom integrations you also need to find web hosting for your code.