I'm not sure if I can edit the page without being disqualified, so I'll put it here. The purple bar represents how much longer you can stop the autofire for. Once it's full, autofiring resumes regardless of your inputs. Its effect isn't super noticeable early on, but after your fire rate increases a bit it can be a massive lifesaver. Also, there is no real 'goal' beyond surviving and getting a high score. This should have been in the game's page, sorry about the confusion.