Rules are as follows:
It can be about anything you want. It's a test on how well you can write about a location, a person, and feelings without resorting to adjectives to describe them, as most nouns and well placed verbs can do it for you. This will only strengthen your writing skills in the end, not hinder them.
There has to be a reason for the scene. Maybe you're coming home from work and upon entering you notice something is not quite right, or maybe you're visiting a place that you haven't been to in years and it has changed for better or worse.
Ok, so adjectives can creep into your writing without even knowing or unintentionally, like saying it was a cold night, or it was a dusty table, cold and dusty being adjectives in those cases - for these we allow as long as they're kept to a minimum as they're not an attempt to flower the description, they are simple, blunt adjectives.
An example of why too many flowery words are a bad thing, and why little, if not none, can make for more powerful writing:
The glorious sun beamed radiant rays upon the beautiful citadel
The sun illuminated the city