If you want to get up close to your subject, closer than you can by setting your lens to its minimum focusing distance, then extension tubes are an excellent way of doing so.
When you turn the focusing ring of your lens away from infinity, the front element moves out from the lens body. The distance between the front element and the sensor (or film) plane is called extension. When your lens is set to its minimum focusing distance, the front element can move no further forward.... more