What the checker looks at
Most players meet your game as a tiny capsule in a list, then read one or two lines, then glance at the first screenshots. The checker follows that path. It looks at the capsule at the size Steam really shows it, the hook in your short description, your tags and their order, the length and layout of the about text, your screenshots and trailer, and whether the page tells people when the game comes out.
Short description
Steam cuts the short description at about 300 characters. The first sentence matters most, because it sits next to your capsule and trailer. Say the genre and what the player does, in plain words. "A cozy farming sim where you rebuild a lighthouse town" beats "An unforgettable journey".
Tags
You can apply up to 20 tags. The first ones count most, so put your main genre and sub-genre first, then the mood and art style, then features like co-op or controller support. Use tags players actually search for. The suggestions under the tag box are common Steam tags for each genre.
Screenshots and trailer
Steam asks for at least 5 screenshots. Show the game being played, not menus or logos. The first four show up most. Keep the trailer short and open with gameplay, because many people only watch the first few seconds with the sound off.
Is my text uploaded?
No. Everything runs in your browser. Your text and images are not sent anywhere.