A brainstorm is a form of group discussion activity. It is one of the most popular idea generation processes. One of the key elements of brainstorming is that the ideas are shared and written down, but are not immediately judged by anyone in the room. Therefore, the process encourages free thinking and creative exploration.
The goal is for as many ideas as possible to be generated. The presupposition is that no idea is too absurd, too untenable.
Any comments (negative or positive) could interrupt the smooth flow of the brainstorming session, and therefore should be kept for the second part of the process.
The evaluation and filtration of the ideas is the second part of the process. It only comes after the brainstorming is completely finished.