What do you mean by View State and what is its role?

As HTTP is Stateless Protocol..View State maintains the state of the page between post backs in an ASP.Net Web Application.

ViewState allows the state of objects to be stored in a hidden field on the page. ViewState is transported to the client and back to the server, and is not stored on the server or any other external source. ViewState is used to retain the state of server-side objects between postabacks.