The smaller the aperture, the stronger the blur. Conversely, at high amount of aperture, sharpness will be higher. All lenses have a measure of maximum opened aperture value. It is measured in relative units: f1,2; f1.4; f1.8; f2,2 and so on. The smaller the number, the larger the hole and the faster the lens. Accordingly, this lens can let in more light, which means that you can shoot even in low light conditions.
Even a small difference in the number of measure of maximum opened aperture is noticeable in the cost of lenses with other identical characteristics. For example, a 50mm lens with f1.4 is one and a half times more expensive than lens with f1.8.