The debugger is a tool for browsing and interacting with a paused application thread. Any thread in an application can be viewed in the debugger (or you can use separate debugger windows for different threads). You can consider the debugger a specialized version of the browser that browses paused application threads.
The debugger provides standard debugging facilities like stepping and breakpoints. It also provides a graphical interface for browsing the state of the control stack in a paused application thread, allowing you to examine local variables and arguments in each stack frame.
You can also use the debugger's interaction pane to interact with a paused application thread. Simply enter Dylan code at a prompt, and the code is executed in the context of the paused thread.