The aperture of a lens, quite simply, is the opening through which light passes into the camera. The wider the opening, the more light can reach the camera sensor, which in turn affects the exposure of the image.
Just as the pupil in the human eye contracts in bright conditions and expands in low-light environments, the aperture needs to decrease or increase to achieve correct exposure – that is, a clear image that's not too dark or too washed-out – in different lighting conditions. This narrowing and expanding is done by an array of aperture blades in the lens that move synchronously to adjust the size of the aperture, up to the maximum that the lens is mechanically capable of attaining (which is the number in the name of the lens).
The aperture blades contribute to the bokeh – the aesthetic quality of the out-of-focus areas in an image. The number of blades can dramatically affect how smooth or "creamy" the bokeh appears. Typically, lenses with a greater number of blades tend to render rounder and more pleasing bokeh. The number of blades will be included in the specs of the lens (for example on its web page on the Canon site).