It's an icon which shows that my HTC One is in battery saver mode, which is the single best thing to ever happen to a smartphone. Means that my battery will last 2 full days easily (3 if I'm careful-ish), with absolutely no noticeable difference in performance. It basically shuts down part of the quad core processor when it isn't required, turns off the data when the screen is off (although emails, notifications etc. still get pushed through just the same) and does a bit more witchcraft which all adds up to mean that I never have to worry about the battery dying on me
I think the longest I've ever gone between a charge is 6 days, but admittedly that was with me being very light on usage!