When feedback is "positive", it means that people are saying that they like something. "Negative" feedback is when people complain about something or say that they don't like it.
Note that you can also call feedback "good", but the meaning of "good feedback" is different from "positive feedback". "Good feedback" is anything that helps you to understand the thing that you made. "Good feedback" can be positive, negative, or a mixture of both. For example:
I showed the film to a few friends of mine who are directors, and got some good feedback from them. Most of them thought that it was too long, and one of them said that I needed to improve the audio quality.
So "good" and "bad" describe the quality of the feedback, while "positive" and "negative" describe what the feedback is.