What do the letters of SLR cameras stand for?

They stand for: a: aperture priority, p: program exposure, s: shutter priority, m: manual exposure.

Advantages of SLR camera: there is only one lens, which can be used for both photography and photography, basically solving the parallax problem. When shooting a scene, the light emitted by the object is focused by the lens, reflected by the oblique mirror to the focused image, and then reflected by the top ridge prism. Photographers can observe the scene through the eyepiece, and the images of the same scene are from top to bottom and from left to right. So it is very convenient to observe and focus.