Each PAPI light is like a slide projector with a red bottom segment white top segment. The four lights are set at a different angle, so that when you are on the correct glideslope you see two reds and two whites. The slope set corresponds with the ILS glideslope angle and will lead you to the radar touchdown point, usually 1000' in. If it isn't an ILS runway, the PAPIs are set at 3 degrees.
When you are on the slope, the red PAPIs are next to the runway. That may not seem important, unless you are a CAA examiner. I was asked that in my air law exam, and couldn't remember despite having stared at them regularly for over ten years!