A prototype should save you time! It’s an easy way to discover material, social, or interaction design aspects of your project before you are in too deep. It’s a way of asking questions with the smallest investment of fuss. “Janky” might mean paper cutouts (versus code or animation), role-playing, cardboard models, words. It might involve materials that will not be part of your final output. It all depends on what you are asking. Rapid prototyping allows you to iterate multiple times to get your outputs closer to the questions and intentions you are trying to put into the work.

If you’re feeling lost, here are some resources:
What is Prototyping?
Design Thinking & Prototyping by Mural
22 Ways to Prototype