SharePoint 2010 Developer Dashboard is same as ASP.NET Tracing. Once we built and deployed SharePoint 2010 solution into SharePoint Server there may be Unexpected issues such as which part of the application is causing a delay to load or consuming more resources etc. To nail down and easy to solve these problem SharePoint 2010 provides a Dashboard to developers which can be turned on as required.
what Dashboard provides:
The developer dashboard logs and displays performance statistics related to the code that executed to produce the page. Tracking involves both elapsed time as well as the load placed on the system by queries and tracking of exceptions