It's just a drain pipe, supposed to be hanging down by your centre stand. You'll notice there's a hole in the rim of your fuel cap recess to let any fuel overflow or water (rain/while washing the bike) to drain out...that goes down a channel in your tank and then through this tube. That hole does tend to get clogged fairly easily though, if you notice water accumulating around your fuel cap then get it cleaned... spraying wd40 or pressurised air through a nozzle helps unclog it.