The Debug view is a view provided
by the Debug perspective.
The Debug view shows the running Mathematica kernel and contains a number of tools, described elsewhere, that allow functionality such as interrupting, stepping, and terminating Mathematica.
When Mathematica is halted at a breakpoint, the view also shows the evaluation stack. This contains a lot of important information about the running of your computation. You can interact with the stack by clicking the different frames. These cause other views, such as the Variables view, to update.