Your eyes' aperture is adjusting to let more or less light in depending on how bright your surroundings are. They do this with out you noticing so that you don't go blind on a bright day or walk into a pole in the evening. Your camera also has an aperture and can do the same thing but you have to tell it to by setting the aperture. Wider opening in the aperture = more light coming in ( f/1.4 ) Smaller opening in the aperture = less light coming in ( f/16 )
See the diagram on the left. The "f" stands for F-stop which is a unit of measurement used in photography telling you the size of the aperture opening. The numbers are backwards, f/1.4 means a large opening, f/16 means small.
The aperture setting also controls your lens' depth of field which means the areas of your image that will be in focus. This has a huge impact on your image. Read through this article to understand it...
How aperture and depth of field work