The exposure triangle explains how each exposure setting, shutter speed, aperture, and ISO, correlates with each other and how the outcome of a photo will look.
The shutter speed is how long the shutter will stay open to take in the light of the area in front of the camera. The quicker the shutter speed the sharper the picture, but less light is taken. The slower the shutter speed the more light is taken in making a photo blurry. You want to make the photo blurry when writing with light. Shutter speed is measured in seconds or fractions of seconds.
Aperture is how much of your lens is opened. It is used when trying to focus on one object while blurring out the background giving it a shallow depth of field. It is also used when taking photo of a landscape by giving deep depth of field making it more sharp. You would use an F-4 when taking a picture of a single image of focus. You would use an f-22 to take a picture of an entire landscape getting everything in focus. Aperture is measured in f-stops.
ISO is how light sensitive the camera is when taking a single shot. You want to adjust your ISO depending on how much light there is in the shot. If it is noon and there isn't a cloud in sight you want an ISO of 100, and if you are outside at midnight trying take a picture you would want an ISO of 1600 or greater. The higher an ISO the greater chance there will be "noise" in your photo so you would want to try for the lowest ISO you can get. If you are in a gym taking a picture of an athlete, depending on the lighting of the gym, you would want to use an ISO between 400 and 1600.
understand exposure triangle - aperture, shutter speed and ISO - photography lesson 2