When you press the shutter button on a DSLR, it sets a chain of events in motion. DSLRs contain a mirror that reflects light from the lens up to the viewfinder. When the shutter is pressed, the mirror comes up out of the way of the camera sensor. Next, the shutter (which is a two part curtain) opens and exposes the sensor to light. When the exposure is complete, the sensor curtain closes, and the mirror comes back up into place.