Exposure is the amount of light which reaches your sensor/film. The Exposure Triangle is what we call the three factors that contribute towards exposure.
This is the hole in the lens which light passes through. The aperture is controlled by ‘blades’, that open and close to let more or less light in. You may think this hole is a perfect circle, but you’d be wrong. The blades create a prism of light, which can be seen when a bright light source shines into your lens.
Aperture is measured in ‘f-stops’. Depending on how much light is coming through, the depth of field will be deep (large area in focus) or shallow (small area in focus) and different settings will be suitable for different situations. If you want to take a landscape image with everything in the frame in focus it’s better to have a higher f-stop. And if you want to have a just a certain area in focus with a blurred background you would use a lower f-stop.
The speed in which the shutter, which covers the sensor/film, opens and closes. Shutter speed is measured in time and will generally be less then 1 second. If shooting out of manual or shutter priority, then you’ll notice the speed will vary quite a bit. When shooting sports it will be quite high, it could be anywhere above 1/1000. The high shutter speed stops the subject in the photo from blurring.
ISO is the camera sensors sensitivity to light. By picking a higher number, you can take photos in darker situations. One problem when using this to try to properly exposed images in darker situations, is they will become grainy the higher your ISO gets.