“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.
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seat backs and tray tables
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 (something) in a position
"Place" is very similar in meaning to "put" but usually sounds more formal.
an upright position
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".
(something) must be (done)
This is a form that people use for giving formal instructions. It means that everyone has to do this. For example:
Seat backs and tray tables must be placed in their upright and locked positions.