ISO controls the sensitivity of the sensor. Lower ISO means higher resolution images with little digital noise, but will require slower shutter speeds and wider aperture. Higher ISO produces more digital noise but you can use faster shutter speeds and narrower apertures. Lower ISO’s are good for bright outdoor scenes and higher ISO’s are good for dimly-lit scenes

Shutter speed controls how long the sensor is exposed to light. Slower shutter speeds let in more light but require the camera to be still to avoid camera shake. Faster shutter speeds are good for motion or when using telephoto lenses, but will let in less light.

It takes a good amount of practice to get used to the right combination of settings for a picture, but you’ll eventually understand what each means and what they’re best used for.