What is this yellow tube blowing air? What is this yellow tube (lower right quadrant of the picture) which is blowing air into the airplane while it's at the terminal? You can see the open hatch on the bottom of the jet through which it's connected.
Most likely it’s a compressed air line which connects into the jet’s bleed air system.  This can be used to crank the engines as opposed to using bleed APU air to do it.

Another use (if you’re talking about the line under the jetway) is to provide cool air for cabin air conditioning without needing to run the APU to do so.