What does it mean to use a certain place as exposure in photography?

This is where the center of photometry is. In other words, "taking A as the exposure" means that the exposure of A is generally considered "just right." For example, if the background is bright and you want to take a silhouette of a person, you have to use the background (such as the sunset) as the exposure. Then the person will be very dark and have a silhouette effect. If you want to take a shot of a person's expression, then you have to use the person as the exposure.

SLR operation is very simple, just put the metering point (note that it must be set to manual, not automatic) in the "for exposure" position, press the shutter button, and hear a "beep" sound. Press the shutter button firmly. In the two pictures below, the first one uses the sky as the exposure, and the second one uses the people as the exposure.