Session variables such as tokens, cookies, or hidden form fields are used by application developers to send data between the client and the server. Since these variables are exposed on the client-side, an attacker can manipulate them in an attempt to gain access to unauthorized data or to capture sensitive information.
Burp's Proxy option provides a feature to enhance the visibility of so-called hidden form fields. This feature allows web application penetration testers to determine the level of the sensitivity of data held in these variables. Likewise, a pentester can determine whether the manipulation of these values produces a different behavior in the application.