What is 'code for standby pengers'? How does it work?

I don't know if the 'code for standby pengers' really exists-I was reading 'Found' by Margaret Peterson Haddix, and in the prologue, it said that Angela had to know the code for standby pengers. I don't really get it. So do you need to say the code to standby pengers to give them a seat? I'm confused. Please explain to me! :( thnx!