If you are hand holding your camera and don’t take a photo with a fast enough shutter speed your photo will be blurry. A good rule of thumb is the mm of your lens is the minimum shutter speed you can shoot. On a 100mm lens you shouldn’t shoot any slower than 1/100th of a second 50mm 1/50 etc. Like any good rule of thumb, the rule can be broken. Resting the camera on a fence, leaning the camera against a wall or using a tripod will let you use much slower shutter speeds. If you choose to use long shutter times, stars can appear to leave a trail across the sky. Waterfalls can take on the appearance of tissue paper. Just remember whatever moves in your photo will be blurry.