Lenses are featured with an iris inside them that can be turned and closed to control the amount of light that hits the sensor. The aperture is measured in f-stops, with lower values meaning a larger aperture and therefore more light.
Controlling the aperture is an essential technique to control the light exposure and therefore the brightness of your shot. However the aperture also plays a key role in controlling the depth of field that is in focus. The more light you let in by lowering the f-stop value the thinner the distance between the planes that define what is in focus in the image.