It's basically a feedback score you are given when you successfully complete trades with people on the site. You can get scores of good, neutral and bad.
You know you can trust someone who has a to of good feedbacks!
If someone has a few neutrals you can read why, people give reasons for them, so maybe they took 7 months to complete a trade! So they got a neutral or something.
Negative or bad feedback means to be weary of the person if you don't know them. Maybe they messed up REALLY bad, but you can check and see if they're okay if they have more good ones.
Like say 6 months ago I had a negative, but now I have +20 positive itraders! It kind of means I redeemed myself later. And sometimes, say you were that guy who made a trade received pins and couldn't send for a long time and had bad communication because of say, bad life circumstances. If you go back and start to make feedback and send the pins or what ever you owed you can sometimes have the negative turned into a neutral :3
On the left of the screen under your name and stuff you can see where it says feedback score, if you click on the number you can go to the page and see your feed backs!
http://disneypinforum.com/itrader.php?u=4143
This is my page, you can check out mine!