DSLR is an abbreviation of the Digital Single Lens Reflex which is a kind of the reflex camera. When the light comes through the lens, it will hit the 45 degree mirror in the camera and reflects through the ground grass to the penta prism in the viewfinder upon the camera, and refracts to your eye. Because photographic lens is convex lens, so it gives a real image that can appear on screen but upside down. When it reflects to penta prism, the refraction in the penta prism will turn the picture to the normal position. So what you see from the viewfinder is the same image as in front of you. When you press the button to release shutter, the mirror will turn up and cover the ground glass that will blind you for a moment while letting the light goes through to record on the sensor at the back of the camera, the mirror will turn down to the right position afterwards.