That's not a "mode", it's a Perspective. Eclipse perspectives are basically a set of windows and tool selectors, and are one of the core concepts of working with Eclipse. There are a "java (editing)" perspective, "debug" perspective, a "resource" perspective, a "team synchronizing" perspective and quite a few more. Plus, if none of them suit, you can create your own perspective or use one of the third-party plugins such as the "hibernate tools" perspective.
To change to a different perspective, you can use the Window menu, certain navigation keys, or one of the Perspective toolbar buttons.
An IDE is no substitute for an Intelligent Developer.
As a side note, just because you're in the Debug Perspective doesn't mean that there are any debug sessions active. For that, look at the Debug window. You can add this window to any perspective, and changing to a perspective that doesn't have a debug window doesn't stop or pause any debugging. To stop debugging, use the "stop" button or right-menu option in the Debug window. Or tell the application being debugged to exit itself.