WHAT DOES MAPPING IS CURRENTLY BELOW SITE STANDARDS MEAN?

It means that your mapping is currently not of a high enough standard to pass the quality required for game submissions. In order for our site to not be chock full of bad-looking, badly-made games we have certain standards that we require games to be over before they are added to the site. You can review some of those standards in the links you were given on the note when your game was denied.

As the note you received when your project was denied said, you need to look into getting feedback and looking at tutorials, learning how to map better.

It's.... pretty self-explanatory, actually. Basically, mapping should be of a decent standard and show some effort put in to the creation of the game in question.

These kinds of maps are usually denied:
- Bad use of tiles
- Bland, empty maps

Honestly, the idea of a game page is to advertise your game, so show off some of the more interesting, better images you have to offer. A black screen with a sprite and some text doesn't really cut it, nor does a large room with a bed in the middle and cupboards up on the walls.

Like said, check out some tutorials, make use of the screenshot thread and practice your mapping a bit more.

That you should have submitted it two years ago.
Now you can either follow Liberty's advices or show your pictures to the others in screenshot survival thread. Also I lead Maps Weekly!, where you can practice with others on regular basis. It's on vacations right now as i don't do any interesting maps right now.