It is 1st request to the page

IS POST BACK

Post back is when we send page to server for some processing and not rendering first time.  Is Post Back is used to check weather page is rendering first time or requesting for server task

private void Page_Load()
{
             if (!IsPostBack)
               {
                      Function();
                }
}

In above example if page is rendering first time then function()  will execute and if page is rendering not first time then the Function() will not execute .

Gets a value that indicates whether the page is being rendered for the first time or is being loaded in response to a postback.

The following example shows how to test the value of the IsPostBack property when the page is loaded in order to determine whether the page is being rendered for the first time or is responding to a postback. If the page is being rendered for the first time, the code calls the Page.Validate method.

The page markup (not shown) contains RequiredFieldValidator controls that display asterisks if no entry is made for a required input field. Calling Page.Validate causes the asterisks to be displayed immediately when the page is rendered, instead of waiting until the user clicks the Submit button. After a postback, you do not have to call Page.Validate, because that method is called as part of the Page life cycle.

private void Page_Load()
{
    if (!IsPostBack)
    {
        // Validate initially to force asterisks
        // to appear before the first roundtrip.
        Validate();
    }
}