SLR is an abbreviation for Single Lens Reflex. It simply means that there is only one lens and that you via a mirror look through the lens that sit on front of your camera. This is as opposed to a rangefinder camera, where the viewfinder gives you a preview of the scene based on another lens that what the camera will be using to capture the image. Hence only a single lens in a DSLR.
The mirror flicks away when you take the shot (and it makes a noticeable noise). This is why you will see the viewfinder go black during the exposure – the light is sent to the sensor rather than the viewfinder.
The D in DSLR is Digital, as your camera has a digital sensor rather than a piece of film. So it is Digital Single Lens Reflex camera or DSLR.
A mirrorless camera (ML) has no mirror and simply sends the light directly to the sensor, that then via a small electronic viewfinder (a small TV!) presents the preview for the photographer in real time.
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