It was a game mechanic I was planning to include, I've been debating whether or not it's a good idea, and I might get rid of it...
Basically, enemies which survive on-screen for long enough would drop a white point item in addition to the normal blue and gold ones, and getting more than 50% of all white point items in a stage would cause something special to happen. That bar was supposed to show how close you are to reaching 50%. It sounds good on paper, but it's hard to balance and I don't really think it's that fun.