TraceBackward is an option for Trace and related functions which specifies whether to include preceding expressions on the evaluation chain that contains the pattern form sought.
TraceBackward -> True includes the first expression on the evaluation chain that contains form.
TraceBackward -> All includes all expressions before form on the evaluation chain that contains form.
TraceBackward allows you to see the previous forms that an expression had during an evaluation.
See The Mathematica Book: Section 2.5.10.
See also: StackComplete.