What is the purpose of Hidden Fields in Vtiger CRM?

Let’s understand this better with the help of an example.
Assume you share a sign-up web form link with your leads through multiple channels, such as email campaigns and Facebook lead ads.
When a lead signs up through the web form, you want to know whether they came from email campaigns or Facebook ads. To get accurate information on the source of lead information, you can use Hidden Fields.  
Hidden Field values are dynamic. You can map a Hidden Field to a URL parameter to capture different values. You can also set up override values that will be used when the parameter value is empty.

Note: If you are learning about Vtiger webforms for the first time, make sure you read this article first.