The exposure triangle is a common way of associating the three variables that determine the exposure of a photograph: aperture, shutter speed, and ISO
ISO
In Digital Photography ISO measures the sensitivity of the image sensor. The same principles apply as in film photography – the lower the number the less sensitive your camera is to light and the finer the grain. ... By choosing a higher ISO you can use a faster shutter speed to freeze the movement.
Shutter speed
Shutter speeds are measured in seconds, or fractions of a second. For example, a shutter speed of 1/100 means 1/100th of a second, or 0.01 seconds. This is also known as the "exposure time", because it's the amount of time the sensor is exposed to light.
Aperture.
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.