Enabling tracing at page level gives you a bunch of information that can be useful while debugging your application.Tracing helps understand which control uses more view state, start/end of PreInit, start/end of Init, start/end of Render, etc. This information appears at the bottom of the page. By default page level tracing is disabled.
Tracing can be enabled at Page Level and also at Application Level
To enable Tracing at Page Level select DOCUME...Full Article.