In terms of map submissions, here's how it seems most players calculate that:
-1 - Its... not a good map. Usually there are multiple things missing from the submission, like the theme is unoriginal AND the build doesn't look good, or the parkour is too hard AND it's all floating. There's just not really a good chance it'll be accepted.
-0.5 - It's not good now, but it has potential to be better with some changes. Something isn't good but it's fixable, like the parkour is too hard or the build is too plain. The theme is good, though, so get working.
0 - It's ok. Some things are good, but others are not. This is usually a way of saying that you're neutral now, but you want to give a higher rating. Similar to -0.5, but the issues are not drastic.
+0.5 - Good work! There's a good chance this map will be accepted, because the theme is good and the build probably is too. However, there's just a little thing that needs to be fixed in order to wrap it up. Maybe there's an exploit somewhere, or an impossible jump.
+1 - It's a good map. Theme, build, and parkour are all original and well-done. I hope this gets accepted! Nice job.