You can think of aperture as being similar to the iris within your eye. It controls the amount of light that comes into the camera reaching the film or photosensitive chip. Different lenses support different apertures, some being able to open up much more than others. You’ve probably seen “f” numbers on lenses. These tell you the size of the diameter of the aperture in terms of the focal length, e.g. f/4 is an aperture of diameter 1/4 of the size of the focal length. Another thing to bear in mind is that it is actually the area of the aperture, rather than than its diameter that controls the amount of light. What this means in practice is that f/2 actually lets in 4 times as much light as f/2. Also, note that brightness reduces as the focal length increases. This is because the intensity of the light is inversely proportional to the square of the distance it travels. So, for two lenses that have the same aperture size, the lens with the shorter focal length will be brighter.
ISO is a measurement of light sensitivity for the film or photosensitive chip in your camera.
If you have a bright lens then you can work with lower ISO settings and you’re not so dependent upon the sensitivity of your camera/film.