The aperture is a variable opening in the lens that controls the amount of light that reaches your camera’s sensor in a given time. . It, in conjunction with the shutter speed and ISO setting, determines the exposure of the photo. Given a consistent shutter speed and ISO setting a narrow aperture will let in less light and a wide aperture more light.
In addition the aperture affects the amount of the photo that is in focus, called the depth of field. With a wide aperture only objects close to the focal point will be in focus (shallow depth of field) while with a narrow aperture objects further away from the focal point will be in focus (deep depth of field).
In automatic mode the aperture is controlled by the camera.