An aperture is an opening or iris in your camera lens that allows light to hit your sensor. The size of the aperture is measured in f-stops. For example, an aperture of f/1.8 is incredibly wide and allows light to stream into your camera. It's perfect for nighttime or low-light photography. However, at midday, a smaller aperture of f/16 will usually work, because there is more ambient light.
You may have noticed that a wide aperture will often blur your image's background, and a small aperture will make your photo crisp throughout. That's because aperture has an impact on depth of field - how much of your image is in focus.
This can lead to some incredibly creative effects. For example, landscape photographers typically use small apertures to keep their scenes in focus. But what if you were to use a wide aperture to create a dreamy, mystical effect? That's a stylistic choice that only the photographer can make. I encourage you to experiment with your aperture for optimal results - some incredible photos can be produced with unconventional aperture effects.
Take the example of this image. Though it is a landscape photo, I used a fairly wide aperture of f/5.6 and a low perspective to blur the foreground. The overall effect is much more dynamic than it would have been with a 'regular' aperture of, say, f/11.