ISO sensitivity equals film speed in analog photography.
The smaller the ISO value, the less sensitive the camera film, or sensor, is to light. The higher the ISO value, the more sensitive the camera film, or sensor, is to light. With a high ISO number, it is possible to take photos in the dark.
The ISO value has an impact on the exposure time (shutter speed). If the aperture stays constant, and you double the ISO value, for example, from 100 to 200, the exposure time is halved (and the shutter speed is faster).
Most often, you get the best possible result with the auto ISO setting.
If you use a high ISO value, such as 400, 800, or 1600, the photos will include more noise than with lower ISO values. However, the exposure times are shorter than with lower ISO values. You also get less blurring of a moving subject, and possibly also less blurring because of camera shake.
With low ISO values, such as 50 and 100, you get better image quality, that is, less noise and better dynamic range, than with high ISO values. However, the exposure times are longer than with high ISO values. You also get more blurring of a moving subject, and possibly also because of camera shake.
To change the ISO setting, make sure you are in the Creative shooting mode, then select the ISO icon on the left.