Shutter speed is ‘the amount of time that the shutter is open’. In film photography it was the length of time that the film was exposed to the scene you’re photographing and similarly in digital photography shutter speed is the length of time that your image sensor ‘sees’ the scene you’re attempting to capture.
The more slower you put your your shutter speed, the more blurry the picture. The larger the aperture, the more blurry the background.
Shutter speeds are measured in seconds, or fractions of a second. For example, a shutter speed of 1/100 means 1/100th of a second, or 0.01 seconds. This is also known as the "exposure time", because it's the amount of time the sensor is exposed to light. Most cameras offer a wide range of shutter speeds, starting at just a few thousandths of a second and going up to several seconds. SLRs also have a "Bulb" mode where you can hold the shutter open for as long as you want.