The Problem Viewer is a dialog that displays issues to the developer that prevent the application from running correctly. When there is a problem in an application, a red notifier saying "Problems" will appear in the top-left corner of the editor. Clicking on this will bring up the Problem Viewer.

Both compilation issues and runtime issues are displayed in the Problem Viewer. What makes the Problem Viewer so helpful is that double-clicking on a row in the Problem Viewer will take you directly to the location of the problem. You don't need to go searching around for the location of the error.

Below is a view of the Problem Viewer in an application I set up to be broken intentionally. You can see that there are a lot of issues and the Problem Viewer allows me to see exactly which classes and methods they're coming from.