Guile's interactive debugger is a command line application that accepts commands from you for examining the stack and, if stopped at a trap, for continuing program execution in various ways. Unlike in the normal Guile REPL, commands are typed mostly without parentheses.
When you first enter the debugger, it introduces itself with a message like this:
This is the Guile debugger -- for help, type `help'.
There are 3 frames on the stack.
Frame 2 at standard input:36:19
[+ 3 #\s]
debug>
“debug>” is the debugger's prompt, and a reminder that you are not in
the normal Guile REPL. In case you find yourself in the debugger by
mistake, the quit command will return you to the REPL.
The other available commands are described in the following subsections.