Aperture is the round opening in your camera lens that you make larger or smaller to control how much light you want to let enter your camera.
The size of the aperture is measured with F numbers, for F Stops. Different lenses will have different ranges.
The primary use of aperture is to control the depth of field. How much of your image do you want to be in focus.
In this image, the F stop is 1.4. By choosing a low number (a wide aperture) this enables us to focus on one element in the image.