ViewState is the mechanism that allows state values to be preserved across page postbacks.

Because of the stateless nature of web pages, regular page member variables will not maintain their values across postbacks. When we need a page variable to maintain its value across page post backs, we can use ViewState to store that value. Values stored in ViewState will be serialized and sent to the client browser as the value of a hidden form input.

This single hidden field contains all the viewstate values for all the page controls