When a Page is requested, it raises various events which are considered to be part of it’s lifecycle. I keep the visual representation created by Peter Bromberg handy with me.
The AutoEventWireUp property when True, automatically wires up some of these built-in events in the Page life cycle to their handlers. This means that you do not need to explicitly attach these events (using the Handles keyword, for instance, in VB).
Examples of these built-in events would be Page_Init and Page_Load.
If you set AutoEventWireUp to True and provide explicit wiring up of the EventHandlers, you will find them being executed twice! This is one reason why Visual Studio keeps this attribute set to false by default.
Edit: (after Chester89’s comment)
It is useful to note that the default value of the AutoEventWireUp attribute of the Page is true, while the default value of the AutoEventWireUp property of the Page class is false