What does it mean to have undefined symbols?

There are no errors in the code files themselves and I am NOT using any external libraries.

I DID add a **`typedef NS_ENUM`** prior to this linker error occurring.

Where do I add this `-v to see invocation?`

Here is the error message:

```
Undefined symbols for architecture x86_64:
  "_OBJC_IVAR_$_UIViewController._parentViewController", referenced from:
      -[PEPI_LessonController setParentViewController:] in PEPI_LessonController.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

```