i see that everyone is posting ppl are "standing by" waiting for an open seat on a plane. what does that mean? can anyone just go to the airport and stand by? hoping for seats to be open and get discounts? i read that most planes, there are ppl waiting behind for "stand by" and if they have open seats, they can fly. see many ppl stand by for first class too. if someone can explain that would be great. thanks advance