If the user does not get enough feedback they will probably end up hating the application. Flask provides msesage really simple way to give feedback to a user with the flashing system. The flashing system basically makes it possible to record a message at the end of a request and access it next request and only next request.
This is usually combined with a layout template that does this. Note that browsers and sometimes web servers enforce a limit on cookie sizes. This means that flashing messages that are too large for session cookies causes message flashing to fail silently. It is also possible to provide when flashing a message.
The default category if nothing is provided is 'message'. Alternative can be used to give the user better feedback. For example error messages could be displayed with a red background.
This is useful if you wish to render each category in a separate block.