The term "slr" likely refers to "SLR" which stands for "Single-Lens Reflex." It is a type of camera that uses a mirror and prism system to allow the photographer to see exactly what will be captured through the lens. When you look through the viewfinder of an SLR camera, you are seeing the exact image that the camera will take, which is a significant advantage for capturing photos accurately.
The term "Single-Lens Reflex" derives from its mechanism; "single-lens" refers to the use of one lens for both viewing and taking pictures, while "reflex" refers to the mirror that reflects the image into the viewfinder.