The term "enum" is short for "enumerated list". It is a common programming term that simply means a user-defined list. You would use an enum if you would like to create a variable that has a list of options or states.
Creating an Enum in StoryFlow:
The enum editor is located in the editing drawer on the far right of StoryFlow. You access it by clicking this button:
Watch this video to see how to create an enum:
Video