When photos were taken on film it was rated by ISO. For ex: 100 would be a daytime and 500+ would be more suited to night. We still use the term ISO to measure digital sensors sensitivity to light but now we can change it in camera without having to change the film.
The consequence of increasing the sensitivity is digital noise. Which presents itself as small colored dots.
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 thumb 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. Many seconds if you choose, stars can appear to leave a trail across the sky. Waterfalls take on the appearance of tissue paper. Just remember unless everything stays still in the photo it will be blurry.