It shows you some images of body sizes and you choose the one that you think most closely resembles yours. You enter your measurements and some other info and it calculates how accurate your perception is.
There is no real science here and no guarantee of any kind of accuracy. But it is interesting and fun.

(FYI. It asks for your location and assumes you're in the UK. If you're not just make something up. I don't think it really matters.)
My evaluation was off by 17 1/2%. I think I look about 30 pounds heavier than what I really weigh! Woa.
I totally believe that, though. Just the other day I was telling a woman at work that I was sorting through some too-big clothes to get rid of and she asked if she could go through them before I donated them to Goodwill. I was kinna shocked that she thought they would fit her because I was sure she was smaller than me. But I asked what sizes she wanted and it turns out that the too-big clothes would fit her.
The gadget also determines your body type based on your measurements. My body type is a "perfect strawberry" (aka inverted triangle) which is exactly the result I expected.



