The Debug panel provides a list of all debug tools currently in use for the Blueprint, such as Breakpoints and Watch Values. During Play-In-Editor, it also provides access to the Execution Trace, showing you each node execution taking place within a given Blueprint.
For more information on debugging Blueprints, please see Blueprint Debugging.
Interface
The interface for the Debug panel changed depending on whether or not you are playing or simulating in the Editor.
When not playing or simulating:
When not playing, the Debug panel lists off any watch values and breakpoints within your current Blueprint.
When playing or simulating:
When playing or simulating the Debug panel shows debug info, as well as the Execution Trace, which shows how long each node is taking to execute.