It is the what you have your aperture, ISO, shutter speed set on and how they work together to make an amazing photo. It is use a lot of the time because so you have the right setting for the photo.
Shutter speed or exposure time is the length of time when the film or digital sensor inside the camera is exposed to light, also when a camera's shutter is open when taking a photograph. When taking a photo in a dark room you want to have a higher shutter speed. When you use a short (fast) shutter speed you freeze the object in motion, when someone is riding a bike use a fast shutter speed and you can capture the bike frozen but yet it still looks like it is moving. For the background to be blurry zoom in to your object and have a low shutter speed.
How big your camera's lens is open to let in a certain amount of light. It's measured in f-stops (f/2.8) (f/4) (f/5.6) are examples. The bigger the opening the more light and the smaller the depth of view, where smaller the opening less light and the bigger depth of view. So when taking a photo up close use a bigger aperture so that it is up close.
The level of sensitivity of your camera by available light.The lower the ISO number the less sensitive it is to light, where the higher the ISO number the more sensitive it is to light. When using higher shutter speed photographer use a higher ISO number. It also has to do with the graininess of the photo. When it is sunny out you want a low ISO like 100 so you can use what light there already is. When it's dark use a high ISO like 3200 so the camera has a little light but doesn't reflect. In a gym with moving athletes high like 3200 for the ISO
The 3 Basics of Exposure & Photography