>>660358ISO is the light sensitivity (and thus image noise).
Aperture is depth of field (and how much light comes in).
Shutter speed is how long the aperture stays open (and thus motion blur).
It's like a magical triangle. Your subject gives you constraints you can balance out elsewhere.
Ex: A huge crowd needs large depth of field, meaning small aperture, meaning you need slower shutter or higher ISO to get enough light in.
Ex: A car race needs very high shutter speed, meaning you have to open up the aperture or increase the ISO to get enough light in.
The brighter your lighting, the less relevant are the constraints as even with low ISO, small aperture and fast shutter enough light is present. Photography is all about light (photo-graphy means "drawing with light").
>>660396Practice and awareness. We tend to tunnel-vision on the subject we want to capture, completely zoming out what goes on around. Crooked angles, garbage or dirt, harsh lighting or distracting colors. I can't tell you how often I thought I had taken the perfect picture only to look at the RAW and realize it was unusable because some horrific shit took up 30% of the background I hadn't even noticed on location.
Ex: Doing a nude shoot outside you tend to miss minor details like discarded underwear in the grass. In the moment it seems like the least of your worries as you deal with lighting, camera, nature and a model, but it can ruin your shoot. Especially bright white cloth will absolutely blow out your histogram and immediately pull the viewer towards it, while you probably don't even notice it in person.
You capture a tiny rectangle of a huge panorama and a tiny moment in the endless torrent of time. Compared to what you experience, the photo is just a tiny slice. But your viewer has all the attention and time in the world to dissect it.
