It is a state management technique in asp.net. ASP.NET Viewstate is preserving the data between the requests or postbacks and stored in hidden fields on the page. if you open the source of the aspx page you will see like
<input type="hidden" name="__VIEWSTATE"
value="dDwyMTQxMjc4NDIxO3Q8O2w8aTw4Pjs+O2w8dDwfg==>
To disable viewstate in the application we need to configure in page for particular page only or web.config file for application level
To disable viewstate in page level
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="..."
MasterPageFile..." Inherits="..." EnableViewState="false" %>
Add the following line in web.config file to disable the viewstate in application level
<pages enableViewState="false" />