The Debugger tool provides a number of important facilities for inspecting programs.
Common Lisp, like most programming languages, uses a stack to store data about programs during execution. The Debugger tool allows you to inspect and change this stack to help get your programs working properly.
You can use it to trace backwards through the history of function calls on the stack, to see if the program behaves as expected, and locate points at which things have gone wrong.
You can also inspect variables within those functions, again to verify that the program is doing what is expected of it.
The Debugger tool also allows you to change variables on the stack. This is useful when testing possible solutions to the problems caused by a bug. You can run a bugged program, and then test fixes within the Debugger tool by altering values of variables, and then resuming execution of the program.