In Dialogflow ES, a webhook allows you to implement custom programming logic so that you can do things like read/write from databases etc.
In other words, it connects your chatbot (the user interface) to your backend (wherever your data resides).
For example, you can save your user input to an Airtable database by writing some webhook code.
You can create a webhook in the Fulfillment section. See how to create a simple Python webhook.