what is DashBoard?
The Developer Dashboard is an instrumentation framework introduced in Microsoft SharePoint Foundation 2010. Similar in concept to ASP.NET page tracing, it provides diagnostic information that can help a developer or system administrator troubleshoot problems with page components that would otherwise be very difficult to isolate. For example, a developer can easily introduce extra SPSite or SPWeb objects into his or her code unknowingly or add extraneous SQL Server queries.
In the past, the only way to debug performance problems caused by the extra overhead of these instances in code would be to attach a debugger to the code and monitor SQL Server Profiler traces. With the Developer Dashboard, a developer can identify this type of problem, either programmatically by using the object model or visually by looking at page output.
Although performance issues and resource usage information is available in the Unified Logging Service (ULS) logs, interpreting the raw data can be very time consuming. With the Developer Dashboard, all the related information is correlated, which makes identifying these types of issues much easier.
The Developer Dashboard is always off by default. To make it viewable, you must enable it, by using STSADM, Windows PowerShell cmdlets, or the SharePoint Foundation object model.