What do the A and M on the SLR lens mean?

A on the lens stands for autofocus mode and m stands for manual focus mode. That is, the focus motor of the lens works in non-working switching.

SLR is the most popular viewfinder system, which is adopted by most 35mm cameras. In this system, the unique design of the mirror and prism enables the photographer to directly observe the image passing through the lens from the viewfinder. Therefore, you can accurately see the same image that the film is about to "see". The core of the system is a movable mirror (as shown in the light blue part), which is placed in front of the film plane at an angle of 45. The light entering the lens (as shown by the red light path) is reflected upward by the mirror to a piece of ground glass.

Early SLR cameras had to hold the camera waist-high and look down at the ground glass. Although the image on the ground glass is upright, it is upside down. In order to correct this defect, the head-up SLR camera installed a pentaprism above the ground glass. This prism reflects light many times to change the light path and sends its image to the eyepiece. At this point, the image is upright up and down and corrected left and right. When framing, most of the light entering the camera is reflected upward by the reflector to the pentaprism. Shutters of SLR cameras are almost always directly in front of the film (because they are located in the film plane, they are called focal plane shutters). When framing, the shutter is closed and no light reaches the film. Press the shutter button, the reflector quickly turns up to get out of the way, at the same time, the shutter opens, the light reaches the film, and the shooting is completed. Then, the mirrors in most cameras will be reset immediately.