Reference: http://www.digital-cameras-help.com/aperture.html

Aperture is a device that controls the quantity of light that passes through the lens. It is an iris type mechanism, which shrinks or grows in order to let in less or more light.

Small numbers represent a large aperture, big numbers – small aperture. Most digital cameras do not have this numbers written on their lenses, but they use aperture as part of their construction. It is also the way for you to select aperture priority shooting mode from your camera to control the depth of field.

Picture Source: http://americanmum.blogspot.com/2008/07/get-your-blog-on-apertures.html

This is the picture sample by different aperture value.

What we can see is, while the aperture value is lower (wider aperture), the background become more blurry. Because the background is exceed the Depth of Field (DOF). When the aperture value is higher (narrower aperture), the background become clearer. So, if you want have a blurry background effect, you need a dslr lens with wide aperture to do this.

For aperture, the size of it and the value of aperture is in reverse. Wider aperture has lower value (example: f = 1.4), while narrower aperture has higher value (example: f = 20). Sometimes, this is very confusing for beginner.