First lets get SLR defined: it stands for Single Lens Reflex.
For the photographer it means when you look through the viewfinder you are actually looking through the lens you have mounted to your camera body. Who hasn't looked into the viewfinder and discovered pure black....because the lens cap was still on. The main advantage of a SLR camera is you see pretty much what the image sensor will see when you trip the shutter.
Here's how the image gets from the lens up to the viewfinder:
