A light meter I think tells you how much light you should have in the picture so as not to under expose or over expose the print. I think it has to do with what setting you should have your aperture set on.

I THINK a manual shutter back up just means that if you take a picture set on automatic and you don't like how it came out, you can manually change the shutter speed to your liking so the pic comes out more the way you intended it to.