Well, it essentially makes the entire photo black-and-white (the way it would if you left Colorize turned off but moved the Saturation slider all the way to -100) then adds a bit of colour tint to it.
It's a feature which seems, at a glance, to be very handy, but actually I find I hardly ever use it. It worked nicely here, though. However, I fear it only worked nicely here because your white balance was so appalling. I beg you to take the Raw Class as soon as you can.