“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 announcements. She says:

Seat backs and tray tables must be placed in their upright and locked positions.

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.

(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:

All trash must be separated into recyclable and non-recyclable material.

All reimbursement requests must be submitted by Dec. 8th.

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".