Consider the amount of air its wings must displace in order to stay aloft. An equal quantity of mass at least. It's passing through that air and, partly pushing it down, but also partially scraping it thin over the bowed top surface of the wing (the Bernoulli principle) which creates a pressure differential that lifts the wing, pulling it upward through suction, and thus the plane. That's why the plane must go fast to fly, and why it "stalls" and falls if it isn't moving through enough air. It's also how turbulence affects a plane. Differences in air pressure mean that in pockets of low pressure there isn't as much mass being displaced by the wings, not enough lift so it falls.

Now, it's quite likely that my layman's comprehension of this is flawed. But I'm sure it's entirely possible that someone will correct me soon :3