ISO
ISO is the light sensitivity of the camera. It depends on the area you are in. The lower the ISO on a sunnier day, the more washed out the picture however the lower the ISO on a grainier day, the more brighter the picture. The higher the ISO on a sunnier day, the more brighter the picture were as the higher the ISO for a grainier day, the less clearer it will be. Sometimes, it's not even the weather that will make you adjust the light for the photo, but the type of photo you need. Below it a diagram showing this.
ISO; Auto ISO; 100 ISO; 200 ISO; 400
ISO; 600 ISO; 800 ISO; 1000 ISO; 1600
Worst photo;
The photo, clearly, it's the best one as it was *way* to washed out the ISO, was on 3200, and it was already a bright, sunny day.