ViewState is used to retain the state of server-side objects between page post backs.

In ASP.NET, ViewState is a client-side state management technique used to preserve the state of server-side controls (such as textboxes, dropdown lists, etc.) across postbacks.

When a web page is posted back to the server, ViewState stores the values of those controls in a hidden field on the page. When the page is reloaded, ASP.NET uses the ViewState to restore the state of the controls to their previous values. This allows the web application to maintain the state of the controls between postbacks without requiring round trips to the server.

It’s worth noting that while ViewState provides a convenient way to maintain state across postbacks, it can also increase the size of the page and hence the bandwidth required, especially if large amounts of data are stored in it. Additionally, ViewState is not secure for storing sensitive information, as it can be easily tampered with by the client.