The f-stop denotes the size/diameter of the opening of the lens. This is controlled via a diaphram, very similar to how you eye's iris opens and closes.
f-stop is expressed as f/number, where f is the focal length of the lens. So if you pass you math (no offence ) you will then know that the smaller the number, the bigger the "hole" (aperture). E.g. If you have a 100mm lens, an aperture size of f/2 will measure 50mm (100/2 = 50), and an aperture of f/11 will give you 100/11 = 9.09mm.
The smaller this opening (larger f-number e.g. f/11), the more depth of field you have. That means more parts of your image will be in focus. Conversely, a large aperture (smaller f-number, e.g. f/1.4) will give you LESS depth of field, perfect for blurring backgrounds.
By now, you realise that to make a les with a small f-number (large opening), you will need more glass. And not just more glass - more precision and all that is needed in making that lens, so it costs more.