Doors to Automatic
 

I hear this command when flying and have always wondered what it means.

I am guessing it's some sort of central locking system so the door can't be opened in flight.

Am I right? And does this command also arm escape slides?

Only some aircraft have doors that lock in flight - on most aircraft the doors are not locked but you wouldn't be able to open them due to the pressure differential.

'Doors to automatic' simply means that the slides are armed and will deploy automatically if the doors are opened.

You should also hear "doors to manual" when the plane pulls up to the gate after landing. If you don't, expect some hilarious consequences! :p