“Seat backs and tray tables must be placed in their upright and locked positions.”
You're on an airplane that is about to land soon. The flight attendant is making announcement. She says this.
Seat backs and tray tables must be placed in their upright and locked positions.
These terms are pretty much only used in airline announcements.
"Seat backs" are the back part of a seat.
"Tray tables" are the little folding tables that are connected to the back of each seat. They're called that because you're supposed to use them for holding a food tray.
"Place" is very similar in meaning to "put" but usually sounds more formal.
Being "upright" means standing up. For example, a person that is "upright" is standing on two legs.
The word "upright" is more formal than the phrase "standing up".
This is a form that people use for giving formal instructions. It means that everyone has to do this. For example: