ViewState is a feature used in ASP.NET web applications to maintain the state of web page controls between user interactions, such as page reloads or form submissions. It works by storing data in a hidden field on the page, allowing information like user input or control settings to persist without needing to contact the server. While ViewState helps improve user experience, it can pose security risks if not properly protected, as attackers may try to manipulate the stored data to exploit vulnerabilities or gain unauthorized access.