Edebug is a source-level debugger for Emacs Lisp programs, with which you can:

Step through evaluation, stopping before and after each expression.
Set conditional or unconditional breakpoints.
Stop when a specified condition is true (the global break event).
Trace slow or fast, stopping briefly at each stop point, or at each breakpoint.
Display expression results and evaluate expressions as if outside of Edebug.
Automatically re-evaluate a list of expressions and display their results each time Edebug updates the display.
Output trace information on function calls and returns.
Stop when an error occurs.
Display a backtrace, omitting Edebug’s own frames.
Specify argument evaluation for macros and defining forms.
Obtain rudimentary coverage testing and frequency counts.

The first three sections below should tell you enough about Edebug to start using it.