I have encountered this several times but I am not sure what is it used for.
I googled it but didn't find a real answer that could explain something in detail.

Can anyone give suggestion what to read?

Try reading this.. Google is your friend
http://www.echoecho.com/htmlforms07.htm

type="hidden" specifies that a Form element will not render to the browser.

Pretty useful when you want to pass around information using the HTTP POST method of a form. Since HTML was primarily designed as a display only markup, type="hidden" is the closest thing you can get to a variable that passes information in a programatic way without needing to display them on the screen as a textbox or whatever.

Hope that helps.

thanks for sharing, I have got idea but I want to provide an example later so that you could help to figure out what the hidden input does.

it does exactly the same as any other form element, but you can't see it.

'hidden' is often used in forms to hide properties from the front end user such as item number, customer codes, etc.

Remember, they can be messed with never the less.

As stated in that page I linked you to:

the purpose of the [hidden] field:

To submit information that is not entered by the visitor.