“Stand clear of the closing doors please.”

You're getting into a subway car. The doors are going to close in a minute, so the train driver announces:

Stand clear of the closing doors please.

stand clear (of something)

To "stand clear of" something means to stand away from it. You "stand clear of" things like:

• the scene of an emergency
• falling rocks 
• large machinery
• a crime scene

In a situation in which you want people to move away from somewhere quickly, you can yell:

Stand clear!

Or if you want to say what people should stay away from, include "of ___":

Everybody stand clear of the window.

Stand clear of the closing doors please. 

This is a standard announcement that the train conductor makes on subways and other trains, right before the doors close. It's a warning that people should move away from the door.