Aperture is one of the most confusing aspects of photography for new photographers (and some old hands). In truth, it does not have to be a confusing. The aperture is an adjustable opening inside the camera lens that works very similar the iris in your eye.
When the aperture opens wide (like your eye dilating), more light is allowed through the lens to expose the film. When the aperture is narrow (like your pupil in bright light), less light reaches the film. This works in conjunction with shutter speed and film speed to determine the total amount of light that reaches the film. Aperture size also affects depth of field.