A single-lens reflex camera (SLR) is a camera that typically uses a mirror and prism system (hence "reflex" from the mirror's reflection) that permits the photographer to view through the lens and see exactly what will be captured.

What is the advantage of single lens reflex?

The advantage of the SLR is that the eye sees approximately what the imaging system will record, so that you can freely interchange lenses, use close-up lenses, etc.

Why is it called single lens reflex?

In the reflex design scheme, the image captured on the camera's sensor is also the image that is seen through the viewfinder. Light travels through a single lens and a mirror is used to reflect a portion of that light through the viewfinder – hence the name "single-lens reflex".