There are explanations of the debug functionality in the LoadRunner help. It is publicly available. For the topics you asked about the links to them are currently (version 12.56):

[Call Stack Pane][1]

- This debug pane enables you to view information about the functions that are currently on the call stack of your script.
- This pane is relevant only when a run session is paused.
- This pane is read-only.
- You can double-click any element in the Call Stack pane to navigate to the beginning of the relevant
function/action in the script editor.

[Watch Pane][2]

- The Watch pane enables you to monitor variables while a script runs.
- This pane is relevant only when a run session is paused.
- The current value of the variable. The evaluated value is displayed only when a run session is paused.
- You can Add, Edit, Delete variables in the watch list

[Debugging Overview][3]

[1]: https://admhelp.microfocus.com/lr/en/12.56/help/WebHelp/Content/VuGen/UI/ui_call_stack_pane.htm
[2]: https://admhelp.microfocus.com/lr/en/12.56/help/WebHelp/Content/VuGen/UI/ui_watch_pane.htm
[3]: https://admhelp.microfocus.com/lr/en/12.56/help/WebHelp/Content/VuGen/105000_c_debugging_features.htm