Aperture is how open or closed the iris is on the lens. A wider/lower f stop means more light will be allowed in to the lens, a smaller/higher f stop will allow less light to reach the sensor.
Shutter speed is how long the shutter stays open, meaning how long the sensor is exposed to light for. A faster shutter speed gives the sensor less time to collect the light.
ISO is how sensitive your camera is to light. Th lower the number the more sensitive the camera is to the surrounding light. However a high sensitive setting the more gain/noise it will add to the image, reducing the quality.