In DSLR cameras, the D stands for digital and the SLR stands for "single lens reflex". Most 35mm film cameras with the ability to changes lenses were SLRs. Why? When the image in front of the camera enters
the lens, it comes in upside down. The image hits the mirror on the back side of the camera, reflecting the image upward into a glass prism which then delivers the image to the photographer right side up, hence "single lens reflex". When you take a photograph, this mirror lifts up, the shutter opens and your image is captured by the film, or now, the digital sensor.
Unfortunately, this mirror action vibrates the camera and may produce a blurry image. This is only visible in mid-range shutter speeds, one second exposures for example. To solve this problem, you can lock up your mirror prior to the shutter opening to prevent any added vibration due to the movement of the mirror. You will need to consult your manual for instructions. If your camera lacks this capability, but has live view capability; the easy way to lock your mirror is to simply use live view. Why is live view the work around? Because in live view the mirror is locked up to so that the sensor can display the image. And if you have anything but a DSLR, there's no need to worry as there is no mirror.