With a dSLR camera, once you press the shutter button down all the way many things happen very quickly. First the mirror flips up. At this point to keep the digital sensor from being exposed to light it is covered by the shutter curtains. They are called shutter curtains because in older cameras they were actually made of fabric and opened and closed like window curtains. Now shutter curtains are made out of metal or plastic and move up and down instead of side to side. After the mirror flips up, the first shutter curtain drops down, exposing the digital sensor so it can record your image. After the sensor has been exposed, the second shutter curtain comes down to cover the sensor back up. The mirror flips back down and the shutter curtains reset back to their original position, ready to take the next shot.
For a video of the real thing, here is a great video of shutter curtains opening and closing in slow motion by the Slow Mo Guys.
For more helpful information check out How to Get Off the Green Auto Setting on Amazon. It is available in print and Kindle format.