As in the MVP or just the general concept, becuase I often see submissions which simply promote an upcoming product or service rather than display a final product itself.
1. Splash page saying what you will do(not effective on HN)
2. Website doing what you say it will.
this doesn't necessarily need to be software powered on the back end. it really depends on what you are working on. if it's a SaaS company for example you could do everything manually and see if it gets any traction before you actually write the code...