What is AutoEventWireup Property, which is found in the top page directive in aspx page?

The AutoEventWireUp attribute, set to true by default, specifies whether or not the Page events are auto-wired. In other words, when the value is true, the Page events are auto-wired, so that Page_Load(), Page_PreRender(), etc. event handlers work with no extra coding. When set to false you must explicitly create set the Page event handlers to methods with signatures that match the event signature.

Visual Studio .NET sets this value to false, and uses the Page_Init() event handler (which is always auto-wired) to invoke an InitializeComponent() method, where the Page event handlers get defind, as well as the event handlers for any server controls on the Web Form.