Here, the feedback is written anonymously. That allows any kind of feedback. For example, people can say out things they’d like to say to you but are afraid to do it, being worried about your reaction on them.
The feedback doesn’t have to be negative. The most important is, that the feedback would be honest and sincere.
If you write feedback to others, it can be anything — what you think is good at the other person that he/she should keep up in your opinion, what you think he/she could improve or avoid in your opinion. Anything honest. Just anything that would be useful that you think the other person should know.
If you get that kind of feedback yourself, that would help you to verify your own self-image that might be true, but also might differ a lot of what other people see in you.
Take sayat.me as a kind of tool to get input information for your self-development.