We have a 2011 silverado truck the choices for driving are P R N D M (it is my husband's truck) it is automatic so what does M stand for?

M possibly stands for manual, look behind the steering wheel you may be able to override the automatic transmission with "flappy paddles"

Nope, that is just it, there are no "flappy paddles"

However there is a +/- on the gear shift along with a little picture of a trailer.. Would this be the "Manual over drive"?

without looking at the shifter I can't help you, perhaps the owners hand book may shed some light on the situation

There will be a +/- on the shifter. It's stands for manual. This allows you to select gears manually

It is to allow you to manually select gears, especially for towing and climbing/descending hills. The button with the trailer is "tow/haul" mode, which allows the transmission to use slightly higher pressures and to rev a little higher before shifting, which helps when towing or hauling a heavy load.