A rubber viewfinder cover is connected to my camera strap. What's it for, and when do I use it? What happens to a photo if I don't use it?
The viewfinder cover is for when you take long exposures on a tripod. You only want light to come in via the lens, not through the viewfinder. Normally your eye covers the viewfinder and keeps stray light from entering the camera that way.
You really only need to use the viewfinder cover if there is bright light behind the camera. The mirror folds up and cover most of the prisma, so if there is as little light behind the camera as in front of it (as you are making a long exposure) it should not be a problem.