How quickly the camera reacts to light is known as the speed. Digital cameras often have several speeds available, known as “ISO” settings, which can be chosen manually or automatically. ISO stands for the International Standards Organization which developed a system for film where a 200 ISO is twice as fast (i.e. it reacts twice as quickly) as 100 ISO. (You may have also heard of ASA or DIN – these were two other standards which were replaced by the ISO.)
Why not just have a fast ISO? There is a trade-off between speed and quality. Generally it is preferable to use a slower speed (say 100 ISO) as it gives a sharper image (a finer grain). But when there isn’t enough light (indoors or at night), you’ll need a faster speed (say 400 or 1600 ISO) and will have to suffer it’s less-distinct image (coarser grain).