The aperture regulates how much light reaches the film and it has an effect on the depth of field, which is the part of the picture in focus. The ability to choose the depth of field is very important for creative photographers.
It is somehow confusing: The larger the aperture the smaller is the corresponding number. Therefore, an aperture of f/4 is a large opening while f/32 means that it is almost closed. There is a linear relation between the shutter speed and the aperture.
When you reduce the aperture the depth of field increases. With most cameras you cannot see that trough the viewfinder, but only on the final picture. Be aware that long focal length of the lens and being close to the object diminishes the depth of field.