Debugger mode is derived from Backtrace mode, which is also used to show backtraces by Edebug and ERT. (see Edebug, and the ERT manual in ERT: Emacs Lisp Regression Testing.)
The backtrace buffer shows you the functions that are executing and their argument values. When a backtrace buffer is created, it shows each stack frame on one, possibly very long, line. (A stack frame is the place where the Lisp interpreter records information about a particular invocation of a function.) The most recently called function will be at the top.
In a backtrace you can specify a stack frame by moving point to a line describing that frame. The frame whose line point is on is considered the current frame.
If a function name is underlined, that means Emacs knows where its
source code is located. You can click with the mouse on that name, or
move to it and type RET, to visit the source code. You can also
type RET while point is on any name of a function or variable
which is not underlined, to see help information for that symbol in a
help buffer, if any exists. The xref-find-definitions command,
bound to M-., can also be used on any identifier in a backtrace
(see Looking Up Identifiers in The GNU Emacs Manual).
In backtraces, the tails of long lists and the ends of long strings,
vectors or structures, as well as objects which are deeply nested,
will be printed as underlined “...”. You can click with the mouse
on a “...”, or type RET while point is on it, to show the part
of the object that was hidden. To control how much abbreviation is
done, customize backtrace-line-length.
Here is a list of commands for navigating and viewing backtraces:
Toggle the display of local variables of the current stack frame.
Move to the beginning of the frame, or to the beginning of the previous frame.
Move to the beginning of the next frame.
Add line breaks and indentation to the top-level Lisp form at point to make it more readable.
Collapse the top-level Lisp form at point back to a single line.
Toggle print-circle for the frame at point.
Toggle print-gensym for the frame at point.
Expand all the forms abbreviated with “...” in the frame at point.